Why is .NET Framework 3.5 not installed by default?

I've always wondered why would Microsoft not have it preinstalled and need people to install it manually? I was under the impression that I always have to install it since sooner or later some program may not run or install if it's missing.

Just want to correct my understanding about this, is there any downside of installing it or any reason you can think of why Microsoft doesn't integrate it by default in the OS?

It's for older programs developed prior to the release of Windows 10. So you might ask why all versions are not included rather than the recent version. Well if a program needs an older version you just install it.

Callender said:

It's for older programs developed prior to the release of Windows 10. So you might ask why all versions are not included rather than the recent version. Well if a program needs an older version you just install it.

Got it, thanks mate.

Just for those that may need to now how to install/enable .NET Framework 3.5 in Windows 10.

Install .NET Framework 3.5 in Windows 10