Because of output size, or to be more specific the relationship between quality / resolution and output size.Ok, but if MP doesn't matter, why don't make it a 3 MP sensor? or even less
At 300dpi (for quality / resolution) a 3mp sensor gives an output size of just 7” long side. A 24mp sensor, at the same dpi has an output size of a very respectable 20”. That’s a big difference and quite a decent size picture for most people.
A 45mp camera has the equivalent output size of 27”, so you can see the diminishing returns of higher and higher mp beginning to kick in.
Then from a practical perspective take note that the larger the output size the further away it is likely to be viewed, which reduces the need for such a high dpi anyway.
IMO for FF sensor size 30mp is the sweet spot.