In a year Nintendo introduced a hybrid console, if you follow Japanese sales and still believe handhelds belong to another planet and mobile to another universe as if it's still 2000, there is no logical explanation except one:
Denial.
Nintendo introduced a hybrid console because it last console failed to sell like they wished and they wanted to merge the console and portable gaming market in one to not face a situation like happened with Wii U.
Switch sales potential is bigger than PS4 because it focus in two markets... portable and consoles while PS4 only focus in consoles market.
What you say is a potential combined market where all game hardware devices focus in both consoles and portable but that is not happening yet because there are console devices like PS4 and XB1... Nintendo is the first to show a hybrid console/portable device... only Nintendo focused in the sales potential of the combined console and portable markets.
Said that PS4 in actual Japan console market is showing that a console can be successful even without the portable market.
In a dumb example...
Group A: 150 potential buyers only buys portables.
Group B: 50 potential buyers only buys consoles.
Group C: 250 potential buyers buys either portables or consoles.
You have clear different markets for potential sales for portables (400 dudes) and consoles (300 dudes).