Because they’re better equipped and it enhances their platform to be a reliable one stop. That said protondb having user comments is incredibly helpful.

Also a point for not relying on protondb, they mark The Finals as a platinum ready to play game but it is a crashing nightmare deserving of silver at best. A common complaint by users there too.

So why rely on protondb if it’s not a guarantee? They have orders of magnitude more data to draw a better picture than the user opt in aggregation of protondb that’s faulty at times.