Why are some APIs paid?

Question

API

I wanted to try the face api from Microsoft, as I realized that it needs a mandatory api key, for this you just need to register. Further, they gave me an api key, and wrote that it was like a trial (for a week in total). Further, as I understand it, you have to pay.
Actually, the question is why some services, for example, VK, do not sometimes ask for an api-key (to obtain some kind of data), but many, even in the same situation, ask, and another question, why are they paid?

Answer 1

It's pretty simple: money.
Or rather, the model with which the company plans (or already earns) money.
VK is free, because. This social network, like many others, makes money on advertising. It is important for her (them?) that you use the API, use the service, stay with them, and then you will definitely see ads ;-)
From the point of view of Microsoft, they have implemented a cool service, their servers and software written by them will work for you , using their ideas, labor and patents and blah blah blah ... Why not take money for it? In a sense, this is their income model.
Still others may have something in between: if it's free, then with advertising, for example. I hope the idea is clear.

Answer 2

Do it yourself - questions will disappear)