I'll just say that maximum hit points per level doesn't mean you're going to get the maximum every time.
Then they shouldn't say it. 'Cause what the hell else does "maximum hit points per level" mean?
It shouldn't be hard to implement a system where you roll for HP, take the average HP or go 'easy mode' and get max HP per level.
All the time, really? Lie. Lae'zel is beastmode given the proper build and weapons.
In fact, having run a number of STR 16 Battlemaster's SOLO builds, I have had nothing to the degree you are asserting or describing.
Furthermore, RNG is simply that RNG, math not involved and don't take having to be a rocket scientist to understand that.
And if you running around in a fighter build with CON 10, you doing something wrong.
If you lower CON, you can increase DEX, which increases your AC and ranged attack stats.
CON helps you tank damage but doesn't help you dish it out. DEX helps you avoid damage AND dish it out.