Its an ultimate ability, that how.
Urgot has to wait until level 6 to be able to execute, even then, the base damage early on is pretty much nothing, Urgot has to work to get you below that 25% health threshold.
If they tailored a execute into a rune, it would be available from level 1, meaning that early fights would be much more onside if someone had that rune whereas others did not.
In Urgot's case, if he screws up early, by the time he gets his ultimate, he wouldn't be able to use it since he wouldn't be able to win fights.
Same thing goes for Pyke, if he screws up early (Which isn't as hard to do compared to Urgot), His ultimate won't be of help since it loses power when not able to execute, you also have to be right up in their face with no blinks aside from flash, his stealth has a pretty large reveal radius so it will be easier to react.)
I will agree however Pyke's ult may be a little on the strong spectrum, Urgot's however has enough counterplay in my eyes (Long drag time, Skillshot, can be stopped if he dies)
I get that you said you have a really strong opinion on this, I just need to be able to say this to Maybe try and help you out.