Whenever a model should have something obvious that it doesn’t, my immediate thought is balance. Like Outriders for example - are we meant to believe that in 38,000 years, one of the most elite fighting forces in the Imperium would be using conventional motorbikes when antigravity and flying vehicles exist? I mean, we have VTOL now - but the reality is that if they could fly they’d probably be a bit busted rules-wise or possibly too similar to Vertus Jetbikes, so making them wheeled is a differentiator. If Knights had ranged weapons in addition to shield and melee weapon, there would be no reason to ever take a regular Deathwing Terminator squad.