First off - selling a million copies is a hard feat for any smaller studio so we wouldnt see a ton of them anyway. But we have gotten a bit more numbers than what you are saying. Some by devs themselves and some by Nintendo. Why they dont promote big sales themselves, I dont know. Some like to be more transparent than others. I dont think high sales number ever made me buy a game myself.. I dont think it makes a big difference really.
Some known numbers below:
Stardew Valley - ~1M as of 180427
Enter the Gungeon - 1M as of 190302
Sonic Mania - 500K+ as of 180424
Overcooked - 500K+ as of 180427
Shovel Knight - 370K as of 180301
And as we got the top 10 best sellings Indies on Switch at GDC earlier this year we know a whole bunch others has sold a significant number (at least 371K assuming SK sold 0 units from march 18- march 19) And as that's clearly not the case it's most certainly higher than that in real numbers.
Other games confirmed to have sold more than SK as of 190321
Graceful Explosion Machine
I would be surprised if more of these has not hit 1M at this point though.. specifically looking at Dead Cells, Overcooked 1+2 that all seem to always be high on the E-shop chart.
As a fan of sales numbers though. I would appreciate more transparency ;)