Because that's both arbitrary (increases cognitive load) and increases intricacy (raises chances of a mistake) for little gain? The user has to remember / learn two buttons to achieve one specific thing, and it provides two opportunities for error. E.g. accidentally sprinting or shooting (button too early/late) could easily wreck a stealth play. Actions with their conteols tied together can also interfere with each others' feel, e.g. do any of them now need a delay to ensure they're not the start of a chord? Chords are fine and dandy, but to be intuitive to players and minimise cognitive burden they should be for related stuff that is mostly or entirely additive. Climb being jump + forward is unlikely to cause issues because the player is likely already using one of them when they need to add the other, and they naturally lead to that action anyway, while requiring both reduces the chance of the action being done by accident.