why are there so many versions of after effects?

The current version of any Adobe app in the program manager is always the one without a version number.  The other ones with version numbers are for users who wish to install previous versions, which can be useful for collaboration purposes and for opening older projects etc.

In this case, it's fairly obvious which version is current because its also tagged with "New".

But I agree with you that Adobe's naming conventions are confusing.  There are two ways Adobe label their software.  The current version is sometimes referred to as After Effects 2020, and otherwise as After Effects 17.0.  They need to make up their mind and stick to one numbering convention.  And the program manager should have the option to only show current versions.