Why there are so many people using Microsoft Office given the existence of some free alternatives (LibreOffice, GDrive, etc.)?

Because it's a user friendly , easy to use and lots of modification, easy to access and synchronized wherever you are , smartphones, tab, laptop as long as your are online ... so there is a lot of features in MS Office that the other free software don't have the next issue is software incompatibilities, although there is some other way to remove the trial of that software is to charck it..

most people didn't learn yet the Microsoft products they are  beginners  and the certified ports are using Microsoft product only

I believe because of functionality and support offered by Microsoft Product.

Microsoft Office is a completely user friendly software and has been used worldwide from a very long period of time. Therefore, the comfort level of using MS Office is a lot more than  using other software. It covers all the areas of work and the support offered by the team is also very good. So I think that is the reason why many people use MS Office inspite of having other free alternatives.

Most people don't know about freeware, so what they got is publicity of the shareware. Also there are good user-friendly features of MS-office

I'm agree with Mr. Ahmed allam

Microsoft being a famous and most common used software and popular amount professional folks