Why file extensions are removed from the URLs nowadays?

There may be many reasons to remove extensions from URLs by various sites these days.
1. The main reason may be to change technology easily. If the URL contains extensions, if there is any change in the technology, the users will also know. If it is not there in the URL changing from one technology to another technology is easy and nobody will notice it.
2. The URLs will look cleaner.
3. It is easier to type URLs
4. The URLs can be remembered easily without a problem.
5. URLs will become more SEO keyword-friendly.

Dr. Rao has answered the question in a nutshell, but allow me to elaborate a bit. The primary reason why the webmasters choose to hide the extensions from the URLs is to hide the underlying technology from hackers and other possible malicious attackers. For example, if a page ends with .html or .aspx, it is easy to find out exactly what (HTML/XHTML and DotNET) were used to develop the concerned website.

In addition to points #1 and #2 mentioned by Dr. Rao above, the extension does not actually convey any sense or meaning to most of the users. Since modern web-developing is highly user-oriented, it is advised that the extension is removed from the URL structure.

Also, let me add something to Dr. Rao's fifth point a bit. If you do not include the extension in the URL, you are saving the space for four or five characters, which may be otherwise utilized. For example, the search snippet in the Search Engine Result Page (SERP) usually shows 64 characters, beyond which it is truncated and does not look good. It is, therefore, a piece of standard SEO advice to keep your URLs within 64 characters (with exceptions, though). The limit is really quite short, you see, and there is no point in reducing it further. Instead of wasting four or five characters in the URLs by writing the extension, you can add yet another meaningful keyword while keeping within the usually prescribed limit of 64 characters.

Languages these days have frameworks which allow masking of the backend. And for that they avoid having the file extension to the URL. I think this was developed to avoid making the language and backend obvious to the security resasons. And also the extension less URLs are pretty easy to type too. So both SEO, Security and the URL specific ease of use is in play here. That's one reason you see how the extensions are removed from the URL these days.

There may be several reasons for the removal of extensions from URL as indicated below -
1) To make the URL look attractive and clean.
2) We can type URL conveniently.
3) It would be easy to remember URL.
4) Making URL more SEO keyboard friendly.
5) We would be in a position to change the technology if we have the inclinations to move our sites from one technology to the other.
Keeping in mind that multiple sites are generated by the the Content Management System ( CMS) that would make URL to look cleaner. For example, instead of / index. php? page = this - is - the widest - page, we could remove the portion index. php? page to look the same cleaner.
Extensions are not required on the web because servers are used to send the type of document as a header. Web pages are served as text/ html image as image / php. This allows the browser to know how to deliver content without resorting to extension to identify the URL containing text, HTML, PDF or image.

The main reason to remove the extensions (like .php or .htm or .html etc) in the files or web page names is to make them more user friendly as well as search engine friendly. In fact one having some programming background can do it oneself by editing the .htaccess file in the system.

At the same time SEO experts believe that it does not matter what extension your file has as regards the search engine optimisation so if it does not matter than factually speaking there is no need to remove the extension. So it boiled down to this that once the pattern of removing is taken up by many website owners then it becomes basically a trend for keeping the site clean without any embarrassing extensions. This trend has already picked up and many others are following the suit.

The site is mainly generated by the content management system that makes the URL with extensions like .php, .html, .aspx, etc. These extensions are basically not needed on the web because the server sends the type of documents as a header. Web pages as test/html, images as image/png or it could be image/jpeg. This lets the browsers know how to render the content without using the extensions like .php, .html, .aspx, etc.

There are many reasons like:
1. Websites are developed using various scripting languages. Like if website URL is http://www.abcd.com/page.pl, where extension .pl is for Perl script. If the URL is like http://www.abcd.com/page.php, .php extension shows that the web developers used PHP for the backend script. These do not look good so a better idea is to remove the extensions.
2. URLs should be easier to remember and cool to be passed to others even verbally.
3. To make URL much cleaner
4. To make easier to search the website

All the extension shows the technology of the URLs and every technology has its own extension and then if anyone wants to change the technology then the extension will also be changed automatically. So, removing extension solves this issue. Also, without the extension, the URLs look good, typing of URL name without any mistake in extension and URLs can be get remembered easily.