Why don't they just get rid of internet explorer ?

This diagram shows the current market shares of all Major browsers. Source: netmarketshare.com

The combined share of IE is about 50% (IE 11 alone counts for 26.9%). When one web browser is enjoying such dominated presence on the Internet, getting rid of it can't be happening overnight. Trust me, everybody wants IE to be gone, even Microsoft itself wants that too, that's why they built a new browser named Edge for Windows 10; but hundreds of millions of computers today are running pre-Windows 10, many of these users have no intention of upgrading to 10 anytime soon, or their hardware just isn't powerful enough to run Windows 10.

As to the why people are still using IE, the reasons are plenty:

1. back in the old days, some websites was developed in a way that requires the web client with IE core to function properly, those sites continued to existed today, and people are still using them.
2. Some people aren't very tech-savvy, they just use whatever that comes with the Operating System.
3. But sometimes you just don't have a choice, when you're using a public computer (e.g. ones in the school library), and the only web browser on the system is IE. What else can you do? You don't have the admin right to install Firefox/Chrome.

I'd also like to point out that, from users' perspective, they can pick their own favorite web browser and forget about IE ever existed; from web developers' perspective, we don't have such luxury, IE is and will continue to be a major player on the web for many years to come, any websites we put together must support IE version 8 or later, whether we like it or not.