we all know that once the colleagues leave company we no longer meet

is it sad

It's generally true which is why it is important to have a healthy social life outside of work.

If you didn't meet them outside of work while you were colleagues, you won't meet them after either

Not really, I still meet some ex-colleagues sometimes after leaving the company. Of course, only those are close.

Colleagues and “work besties” are different groups of people.

Colleague are not you family, I do not entertain after my work.

They are colleagues whom you met through your employer. If they join you at your new employer then you will continue to meet for the same reasons. When my house was being built, I met the architect and builders regularly and now the work is done we no longer meet.