They have these fancy automatics that let you select your gear now.

I know right.
There are DCT's which are alright, but even most autos with manual select have inconsistent shifts, don't hold gear, etc. I've test many of them, and they rarely seem to work right for whatever reason, which I suspect is fundamental to the converter/planetary-clutch design.
That makes no sense. Add power to an AT and you get to each gear faster at (~any) throttle point. Downshift and you make use of the higher RPMs, like one would with a manual.
The main difference is some of us have outgrown the fun factor of manually shifting gears and would rather not, are fine with the car doing it for us when it's a public road instead of a race track.
The one positive thing I find in a passenger (non-towing) vehicle manual tranny is the driver is less likely to be using a free hand to text while driving. Well, lower repair costs too except it's been many years since I had an auto tranny failure so my repair costs have been averaging down towards $0.
Consider the way the throttle correlates to power. When you have 10% more power, that really mean you only need to be at, say 45% throttle instead of 50% for the same oomph. But that difference is only really relevant at the top of the scale, when you can't get past 100% on the original car, whereas 100% on the upgraded one corresponds to 110% on the original.
Now consider how manuals are used vs autos. At least the way I drive and I suspect many mt drivers: use high throttle and short shift at moderate rpms. That means you use the range where the difference is relevant quite often, whereas if you did that in an auto it would downshift.
When you do downshift you get more power, but that's not really the point, since if you're short shifting as usually, you're not really trying to go as fast as possible.