[sbierwagen]

>You want to be as far south as possible, for efficiency reasons

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.

[gpm]

For normal moderately inclined orbits the boost is still greatest at the equator, just always less than for non inclined orbits (until you start launching retrograde).

The soviet launch site was indeed chosen for other reasons than maximizing payload... in particular I think I recall that it was largely motivated by avoiding launching over China, and being in an empty enough part of the world that it's unlikely to kill to many people (despite launching over land).