Disclaimer: I don’t use any googlee services myself.
Because it is free, guaranteed to work as long as they keep it running and marketed well.
Plus since they were early into the game of tech online they have many services that all link together.
There aren’t many that will offer most users so little value for ‘free’.
Most alternatives will have some cost if you want as much space as google provides, either the same as google (user data) or monetary (which I semi agree with, hosting isn’t free and I’d rather pay money than with data). However, not everyone is in a position to pay with money and so data is usually what they pay with.