ADCs actually define what can and cannot be picked moreso than virtually anything else.

Before I begin this, I'm going to say now that the only thing that comes close to defining how mobile the meta must be is Jarvan because of his ult.

The idea of ADCs defining mobility actually comes from duelist ADCs. ADCs like Kalista/Vayne that are very good at dueling, mostly through a combination of tank shredding and being untouchable by most champions stifle non-mobility harder than you guys believe Lee ever could. Picking a tank without a ton of easy-to-land ranged hard CC into either of these two means that you'll do very little to them in a fight. Not only are they difficult to catch, just tanking them for your team also isn't that viable of a strategy sometimes since they're so good at killing tanks. They effectively reduce options down to "burst them very quickly" or "kill their team really quickly and they probably can't 1v5." These types of champions are both obnoxious to play against in general while also creating a meta of bursty champions around them to deal with their anti-tank/anti-bruiser shenanigans. This means that mages can also end up being difficult to pick into them because of their massive mobility. Having a mobile tank shredder will oppress more picks than having just about anything else with lots of mobility aside from an assassin.

Although not all ADCs define what can and cannot be picked, those that do make it one of the most terrible experiences to actually pick what they counter into them.