Are work friends actual friends? Seems to me in the vast majority of cases once a person switches jobs he switches his work friends as well...which happens about once in 3-5 years...

This is my experience as well. Most "friends" only stay friends for as long as they share activities together, and if there is too long of an absence in activities, it will be harder to plan activities or reconnect later down the line. They'll usually drop down to acquaintance level and stay there. Goes for work, hobbies, and more. Friends who stick around for who you are, rather than what you do together, are exceptionally rare. Especially later in life, when everyone is "too busy" for activities outside what they already do.

I hope my experience is the minority, as the increasing loneliness that seems to be present as the younger generations grow older to me indicates the over reliance on work for friendship is the exact opposite of what we need. And on a much darker and more cynical level, it provides an opportunity of social manipulation for companies to keep one tied to their job at a lower wage, or risk crippling loneliness.

I'm pretty sure I could have become actual friends with many colleagues if we weren't colleagues, but I see that meeting them after hours always gravitates to talking about work. Some people are fine with that, to me it's depressing to have to think about work on my weekend.

It depends on how you define "friend."

I don't require that my friends have any interest in me outside of the contexts they wish to be friends with me in, though, and so I do consider work friends actual friends.

Same thing with cycling friends, childrearing friends, and hanging out and drinking friends.

Why should a friend share extra time with me for some reason? In any case, the more time you spend with contextual friends the greater the likelihood you will meet a lifelong friend.