It is absolutely true that a rocket launched at the equator gets the biggest boost from the Earth's rotation, but stuff isn't put into orbit just for fun, it's often because you want it pass over a specified part of the Earth's surface. To pass over CONUS you need some inclination, and the more inclined an orbit is the less assist you get from the planet's rotation. A polar orbit (inclination 90 degrees) has no assist, and a sun-synchronous orbit is slightly retrograde, where you then want to launch as far north as possible, so you don't have to cancel out as much rotation velocity!

The Soviets put their launch site at 45.9 degrees north not because they're bizarrely stupid, but because they'd like their orbits to pass over Russia. Similarly, no Starlink satellite has had an inclination lower than 42 degrees: https://en.wikipedia.org/wiki/Starlink#Constellation_design_...

Going forward, depending on how much interplanetary traffic there will be, you can imagine Guiana Space Centre (Which is currently hosting JWST, destined for a Lagrange point) will see more traffic, but right now almost everything has some amount of inclination.

If launching from an ocean platform, then fresh water seems pretty easy to bring in via tanker. That's less workable at Boca Chica, or on most tiny islands - where ports for decent-sized tankers are scarce.