There's a huge difference between being immobile melee and immobile and ranged

Typically being immobile is a designed weakness for both ranged and MELEE champions alike. But let's get one thing straight: there is a HUGE difference between being melee and immobile and ranged and immobile. Being ranged in itself is a huge advantage for starters and generally, ranged immobile champions will have tons of power in their kits that make up for their lack of mobility while melees typically rely heavily on durability, which durability only lasts for so long.

Being immobile like a Nasus, Garen, Darius, Singed, etc is much more damning than being immobile as a Syndra, Ryze, Lux etc. These champions have long ranged abilities or generally have enough range to keep themselves at a safe distance from ever being in real trouble in solo situations. They've also got plenty of power to disrupt any attempt of engagement on them while also using that same power to pin you down and nuke you. The only issue you have is being ganked and while that is a weak point for you, it's a designed weak point, not a death sentence.

Melee champions who are immobile typically only have one route of viability: some gimmicky style of play. They can't teamfight, that's for sure. Their immobility IS a death sentence in many situations. The best they can do is clean up in teamfighting, or using flash + engaging, which is weak in its own right (though admittedly potentially game changing). But ranged champions can do this too with the benefit of being ranged and not relying on flash.

So please, when you say "yeah but this champion is immobile," please keep it in perspective. If the champion is immobile and ranged, especially long ranged, then mobility for them really isn't that big of an issue. But being immobile and Melee, yeah, have fun not mattering.