It's obvious to me that you're new to traveling because you want to consolidate points with one hotel rewards program. If, like me, you'd been traveling to various places around the world (and not always just the very biggest cities), you would long ago realized that no one hotel program works everywhere.

Want to go visit Iguazu Falls (Argentina/Brazil border)? SPG is the only one that will get you there inside the park, and the only hotel program in one of the surrounding towns is Best Western. Want to go to Salta, Argentina? The only hotel program there is SPG. But some other comparably sized towns in Argentina, the only hotel program there is Wyndham Rewards.

Want to visit the fjords of western Norway? Once you get outside the big cities (and there are no big cities out on the fjords themselves), the only hotel programs you find are Choice and Best Western. And even in moderately sized cities, outside of Oslo, the only additional chain (where you could earn in the US) that you're likely to find is Club Carlson (for Radisson SAS).

Want to stay on points near Stonehenge in the UK? The only hotel program you'll find there is Priority Club.

Want to stay anywhere near Bryce Canyon National Park in Utah? There are two Best Westerns right outside the park, but no other hotel programs within a couple hours around there. When you do finally find some other programs (at least a couple hours' drive away), they're Choice, Priority Club, and Hilton HHonors (because of Hampton Inn), maybe a Fairfield Inn here or there (from Marriott), but nothing from SPG.

The only program of the ones I've mentioned about that I don't have anything collected in is Club Carlson.

And besides that, sometimes in a particular location hotel program X requires tons of points while hotel program Y requires very little, but in another location it might be vice versa! Having all your points in one program means you have to collect even more points than if you have a good enough balance in several programs that you can always choose the hotel in each location which is the best value on points.