Several reasons, the first being tread life, as the all seasons will outlast the all-weathers every time.
The second is because you can get great all-season tires that you can run year round without losing your comfort and performance, saving yourself the cost of a second set of rims, TPMS sensors, and the hassle of change-overs twice a year.
Winter tires are great, everyone who gets winter should use them, if you don't get winter, save your money.
1) Tread Life. This is only the 2nd winter season for my Nokians, so I'm not sure of the tread life yet. However, I have about 5000 miles on them, and my tire spreadsheet is predicting a tread life of about 28,000 miles. This is longer than the Pilot Super Sports will last. Not quite as long as the Primacys (spreadsheet predicted about 35,000 on those before I removed them). Yes, a long-life set of all seasons might go 45K or 50K, but see #2:
2) You're making the argument that a good set of all-seasons will perform identically to the Michelin PSS in the summer and the Nokians in the winter. I'm going to say that is up for a lot of debate.
Potential candidates would be an ultra-high performance all-season tire, including:
Michelin Pilot Sport A/S 3+
BF Goodrich G-Force Comp-2 A/S
Continental Extreme Contact DWS 06
Pirelli P Zero Nero
Bridgestone Potenza RE970AS Pole Position
I will make the argument that none of these tires will deliver the equivalent summer performance to the Michelin Pilot Super Sport
and the equivalent winter performance to the Nokian WRG3.
If such a tire were actually made, no one would buy anything else.
And none of those tires will go 45K-50K miles on the Tesla, either. To get that kind of tread life, I would have to go to a grand touring all-season, such as:
Pirelli Cinturato P7 All Season Plus
Bridgestone Turanza Serenity Plus
Michelin Primacy MXM4
Continental ProContact TX
GoodYear Eagle Touring T0
Those tires won't even come close the the PSS/Nokian combination for performance.