I think that over time our concept of mental disorder has expanded to embrace more and more people...

A concept that was originally reserved for people who needed to be physically restrained to prevent their endangering themselves or others...

Has come to be applied to people who express dissatisfaction with their lives. Those people once would seek assistance from clergy. Now they go on prozac.

Once upon a time you had to have A or B or C to count as having a mental disorder (for example)

Whereas now you have to have A or B or C or D or E or F or G or H...

So... It isn't so surprising that more people would qualify as having one.

Whereas people who had extremely florid delusions for much of their life, depression such that they were catatonic etc probably WERE at a survival / reproduction disadvantage...

It is less clear that a single episode of existential angst constitutes an evolutionary dysfunction.