For example: Nidalee mid/top. Kha’Zix mid. Sejuani top or support. Amumu top or support. Zac top. Graves ADC. Xin Zhao top. And so on.
Actually, Riot often does the opposite: Whenever a jungler is going in other positions, they tend to nerf it. They did it with Nocturne for example.
because junglers have innate sustain or high base stats that is unfair to play against.
viego was played on release on lane and had incredible sustain from minions, it got nerfed then none played him there again.
lillia was kiting top laners so easily and was not like at all.
rammus top can go 0 10 but oneshots the enemy auto attacker once he has thronmail and sorc shoes (baus thing).
leesin had so high sustain like viego from his W and massive wave clear from E to get prio in lane.
noc had massive sustain from minions too and stat checks people till he wins with a one dimensional playstyle.
zac is playable now on top and is so disgusting for how much damage and heal he does from minions.
why do you want champions that are built around sustain to be viable in lanes? some of them have gigantic health regen like VI that can soak a lot of damage an heals it back again.
to clarify, Vi has 10-27 health regen per 5 seconds, which means she can have 120 HP per min with basic regen. this can go really really wrong in mid lane vs mages for example (there was a video promoting this strategy on YT).
junglers have high base stats that allow them to start 0 stats items and can compete with laners who have doran item to some extent. and you can’t balance that to make them viable in lane without tinkering with base stats.
and finally if you want to play a jungler in lane, go ahead nothing stops you.