In order to fairly retain skills from higher levels, you would necessarily have to reduce potency of skills already learned by that level in some weird hodgepodge algorithm to make it comparable.
Like, let's say you have a combo at L90 that does 1000 potency. Say 100 first skill -> 200 second > 250 third -> 350 fourth. The new players don't have that same combo, they only have the first two skills (so they can combo up to 300 potency). Even doing two cycles of this, for the same number of GCDs (four, two times two), they'd be doing only 600 potency for every time the L90 player is doing 1000. And mind you, this is ignoring traits/transforming skills that become stronger versions over the course of leveling.
So what do you do? Make the L90's abilities about half(ish) as potent? 50+100+200+250 = 600? See, can't even just cut everything in half, instead need to manually tweak every skill to match the highest-potency combo the at-content-level players would have. What if the newbies have the third combo skill there? Now you need to account for them doing 650 damage every three GCDs, except you'd have to balance it out with algebra and shit cuz the "GCDs performed" line only matches at 12 (they've done 4 full combos with 3 skills, you've done 3 with 4 skills). You wanna do the math there? Cuz I don't feel like doing so. Now balance out a job with 5 or 6 skills in their full rotation, or alternate branches where the "middle" skills are different (speaking honestly, yes, I can only talk about DRG from experience - but there is indeed a 12345 but there's also a 1ab54. Also 1 becomes a higher-potency skill after doing the last hit in the combo. Also after doing two 1s (modified) you get another OGCD attack that literally doesn't exist for the lower level player - factor that into your math).
You gotta get parity somehow, otherwise every duty becomes a carry by nature (regardless of how you feel about sprouts' general capabilities, there is no denying that if you're doing almost twice the DPS they possibly can, all else being equal and even if they ARE using their skills perfectly.) By its nature, you rouletting is you helping someone else complete their duties at the level OF that duty - usually for actual new players, or lower-level alt jobs. It's fine and well for the latter, the faster they can grind their 10th job to 90 the better; but a new player is not necessarily looking for a carry and/or basically-unsynced run (which your higher potency makes it, albeit maybe you being unsynced a few levels higher vs level cap running a L20 unsynced and you 1HK everything with basic attacks). You are now imposing that on them though, whether they like it or not.
Edit: I guess, true to the thread title, you might call this "lazy"? That they aren't willing to factor in every permutation of every level's skill availability, and how much they need to change the overlevelled jobs' potencies? Like, they already seem to struggle enough not cutting out skills and balancing what they do have - the real lazy thing would be level cap rotations being simplified to 1-2-3 with one or two OGCDs acquired by L50, and then just making you stronger by traits like "skill 1 becomes SUPER skill 1" or "all damage increased by 10%".