First of all, midlane or jungle isnt always there. Raka can ult to save you from the last tick of an ignite. Braum can ult to knock up the entire enemy team to save you.
The ADC has the damage. The support has the utility. The support simply lacks damage unless you're playing a "support" (a mage you put into the bot lane).
If you try to take any support that doesnt have a primary of mage in a solo lane, you are going to get torn a new one. You can try playing standard Braum/Soraka/Leona in the top lane, but you wont get very far unless your jungler camps your lane (which still isn't a good idea because of early plateauing damage).
If the adc leaves, you have a 1/12891283749658716732908746 chance of winning your lane.
I always try. I get yelled at for no wards, then get killed for warding. I set up kills for my ADC in lane, but they don't go for them and finish it off. Some immobile ADC's get locked down and stunned. What am I supposed to do about that early game?
The support lacks the damage but has the utility. Some champions, like ekko, diana, and jhin, have the utility and the damage in order to easily outclass supports.