Well they kind-of are. Even with the exact same casing by the same mfg - like Vittoria - in the same size. I have that - clincher and tubular and also Veloflex. The clincher has material and air between the brake tracks. The bead is fixed. On a corner more of the side tread is used. The tubular, all the air is on top/outside the rim. The tire rolls a bit on the rim. Less tread is needed on the sidewalls in the same turn. Then the tubular has more usable, compressible air in the same size as the clincher, so on significant bumps there is less casing distortion.

I can put 22wide tread (Vittoria) on a 25 wide tubular and be fine for any debris-free cornering while I could not use that same patch on a clincher. Less rubber means lower rolling resistance. So for reasons of more compressible air and the option to use less rubber/case ratio the same exact tires do not need to exist.

That and the whole brake track you mention just makes the lighter setup the tubular one.
I don't really have high performance clinchers to compare but I know they tend to run about a 600g more a set than what I would select in a tubular. I grabbed this alloy clincher and the year raced (on chip seal, cobbles, all kinds of dry junk) tubular to illustrate the difference - both have skewers.

Tubular - thin tread, 25.5mm wide, 50mm profile ready to ride: 800g
FMB Record.jpg

Clincher - (shown below) less tube and tire: 980g
Alloy Clincher.jpg