I can understand why.

At the end, it boils down to personal needs in my opinion.

If you have a few friends, you have time to keep in touch with them all the time. You might trust (or verify) them. You might reach deeper connections. But you are limited to a few peoples opinion.

If you have shallow but more friends, you will get more and more ideas from different people, you might see more diverse people, more life situations, etc. But you may not share deep connections, and deep personal things.

Of course mixture can be done, but as your time is your hardest asset, something else will be left out. So if you meet new people, you skip your deep friends, etc...