"PC Gamers" are people who play ANY game on their PC... and yes, that is HUNDREDS of millions. Hundreds of millions of stay at home parents and grandparents playing Peggle 2, toddlers playing edutainment and Windows Pinball or Purple Place.
However the number of PC Gamers
is roughly equivalent to the number of gaming PCs capable of running current gen games (Series S' power level and above.), where the pool is closer to 100M. There are only about as many people who actively use Steam to play games as there are Switch games in their entirety, and many, though not most, Steam users don't have "gaming capable" hardware and are using it for social features or lighter titles.
Why target that demographic by pandering to them directly with more development efforts when you could convert many of them to simply BUY your console, and now you have a captive audience which is way more profitable per user?
One Nintendo Switch owner makes orders of magnitude more money than a single Mario Kart Tour user.