There's more to it than that.

Nintendo have spent decades meticulously building up a powerful brand image. As such, there are people who are fans of Nintendo the company/brand to an extent you don't see with Sony or Microsoft; other company's individual games have fans, but you don't get Microsoft Fans or XBox Fans so much as Halo Fans. This means Nintendo's games and hardware reïnforce each other; someone who bought a Switch for Mario and Zelda is the sort of person who thinks "Ooh, Fire Emblem!" and buys that, too.

Plus, if you've gone to the trouble of buying a Nintendo console for one or two games, there is a psychological urge to justify your choice, to make it worth your while. So if you just got it for Mario and Zelda, well then you've spent hundreds of euros on a machine to play two games. But if you also get Fire Emblem, that's three games. Kirby brings it up to four. Thus, every exclusive makes the console purchase feel more reasonable.

Then there are sales breakdowns. Nintendo gets 100% of the non-tax price of every one of their own games. To give a concrete example, Donkey Kong Bananza costs €69.99. Nintendo get €56.90 of that (€69.99 less 23% VAT). If they sell it elsewhere, they lose another 30% of the non-tax price in store fees. Thus, if they were to bring Donkey Kong Bananza to Steam, they would only get €39.83 per copy (€56.90 × 0.7). They would thus need to sell 1.43 times as many copies on Steam to make the same money as selling them on the eShop.

That's the big thing right there. Some people for sure decide to go without Nintendo games if they aren't on other platforms. A subset of those people would be willing to buy Nintendo games on other platforms. But is that cohort at least 43% the size of the cohort who just buy a Nintendo console?

But, of course, Nintendo don't just sell their games on the eShop. There are also tons of third-party games, each of which pay 30% of their non-tax price per purchase. If somebody has a Nintendo console, even if those third-party games are on Steam and PlayStation, they might buy those games on the Switch instead. (Source: I have done this. If you own a Switch, so have you). That's another chunk of money Nintendo would lose out on if they weren't selling consoles.

In conclusion, for third-party developers, being multi-platform is good. But for Nintendo, a first-party developer, keeping their games exclusive is the logical choice.