For a variety of reasons, but before I spend effort answering, I'd like to know what this has to do with anything? Unlike Xbox and Playstation, exclusives, or more specifically Nintendo games, are the driving force behind Nintendo systems.

That's a flawed reasoning.

The more quality options you have to chose from, the more interest will inevitably spread out across different titles instead of focusing on one that'll show up nicely on the charts. ("1-2-Switch" didn't sell over a million because it's particularly more attractive than 99% of games on the market.)
And the more super popular casual centric titles are released over time, the more they will dominate the top spots of the charts, regardless of how well the other exclusives (realistically) sell.

Let's say we have 100 gamers who are unsure of which console to buy, but they all want to play GTA Vice City.
It's on both PS2 and XB. So what's the tie breaker?
It can be many different things, but the most common one imo, aside from price, is other games.

So in the end all 100 of them buy GTA VC, 20 of them buy FFX, 8 of them buy Shadow of Colossus, 5 of them buy Onimusha 3, 4 of them buy Ace Combat 5, etc.

GTA Vice City will show up nicely on the charts because it's appeal is consistent across the board with both the casual and the core audience, but while people like you will dismiss Onimusha 3 because it's not on the charts, GTA is not all they ever play. And while taste varies more outside of those super popular titles, those game sales do add up when you put them together, and they are a relevant factor in why people chose a particular platform.

The first half of 2017 was the strongest first half sales wise for PS4 in its history. Notable titles that released during that window were:

Resident Evil 7
Horizon: Zero Dawn
Mass Effect: Andromeda
Gravity Rush 2
Yakuza 0
Nier: Automata
Persona 5
Nioh
Kingdom Hearts 2.8

I'm not saying those exclusives (RE7 and ME:A were multiplatform) were the main driving force behind the sales, but they made a significant difference.
In spite of the fact that several of the above mentioned titles sold millions of copies, they will never appear on the charts.
But do not dismiss their importance because of that. Choice is one of the most important things to have. And that's not a factor that is properly measured by charts.

Which then makes the statement even more bizarre. That could be said of any device with lots of games. Why single out one of them?

And I agree with that. I just wouldn't measure that by looking at what games are in the Top 20 charts for Playstation.

If not sales, what else can you use to measure?