Because Half ORcs already have feature Orcs don't get in 5e. And in 4e as well.
Going "Take an Orc feature and a Human Feature and mix them" deletes that.
It depends on what those features are. Even current RAW, Half-Orcs have:
Darkvision (orc)
Relentless Endurance (orc)
Menacing (skills-Intimidation; human)
and
Savage Attacks, which neither currently have (a granted point), but isn't a far stretch from something like
Savage Attacker (the feat, which variant humans get a feat anyway).
So, a lot depends on how such a system is created. Using my own (above) as a template, the current Half-Orc could easily be re-imagined as:
Darkvision (strong feature - Orc)
Relentless Endurance (weak feature - Orc)
Savage Attacker (strong feature; feat - Human)
Menacing (weak feature, skills - Human)
And would be even more versatile due to the Human component. It isn't
that much of a stretch, really.
Even the current Half-Elf could be done as follows:
Darkvision (strong feature - Elf)
Fey Ancestry (weak feature - Elf)
Skill Versatility (weak feature x2 - Human)
Extra ASI +1 (weak feature extra, see below)
With the caveat that if you select more weak features (as in the double human weak above), you get an additional ASI +1, which is why Half-Elves get +2,+1,
+1 instead of just +2,+1.
It is practically build in already, without much work.
