The easiest way to explain to the total count believers is in market share.
If a company has 90% market share in one country, that's great, but the company that has 50% market share in 3 countries is simply put the more global company even if the company with 90% share has the same revenue. And why? Because it may well have to pay more in taxes and the revenue in that country can be more mitigated.
That's basically the way you have to look at it. Now in tennis terms, dominating at 2 Slams is surely more impressive than dominating at 1. If you happen to grab an equal total but lopsided at 1 event it clearly means you were only able to dominate one event and over time get a favorable draw in the others.
In the Nadal vs. Federer scenario, let's give Nadal 3 more French Opens only so it's 20-20.
It's not just:
1-14-2-3
6-1-8-5
Or the 6-0 at the 5th biggest event (WTF) but here's how they would have performed from their first SF:
Nadal
SF-W-Q-Q-F-A-F-Q-1R-F-Q-F= Somewhat consistent contention overall but no period of dominance with of course 1 title.
W-W-W-W-4R-W-W-W-W-W-Q-INJ-W-W-W-W-W= Absolute dominance throughout with 2 small gaps.
F-F-W-A-W-F-2R-1R-4R-2R-A-4R-SF= Serious contender for an extended period but no dominance with 2 of 3 titles.
SF-SF-W-F-A-W-A-3R-4R-W-SF= Not even close despite 3 seasons participated yielding 2 titles. Last title notably lucky.
Federer we know but to summarize, 5 in a row at two Slams, 3 of 4 at the AO with a very long contending streak and one last back to back run showing longevity.
Then we throw in the WTF and it's a done deal.