ok now I know the reason why you think supports can't carry...
isn't a support... she's an AP marksman...
she scales with Items more than she does levels...
Example: As soon as I grab
for my
I can proc TLD with a Q,E auto. If I started as a support it would take me YEARS to grab even a 
Even the mages going botlane aren't real supports. They just use botlane as an effective "if I accidentally get kill from adc then it's fine. I also have enough utility to work for most adc's"
considering slows, stuns, and roots are all very powerful and common on mages...
that's partially to blame for the state support is in right now.
Anyone not a support main has a very high chance of not liking the role. Whether it's that they don't know the optimums or that they simply don't enjoy the playstyle... being a support is frustrating when riot supports one type of gameplay yet pushes for another.
Everyone should at least know BASIC knowledge of a role. Example: If I main top/jg and have never played mid in my life; I know that a midlaner's job is to waveclear therefore choose a mage like
or
so that I can waveclear properly and I have a simple kit which allows me to do so.
Supports can't carry in a traditional way.
How a support like
would carry is by keeping her horrible team alive.
Ex: you have a team of
(mid)
(you)
Your wood-division
decides to flash ult into enemy team. You flash and end up using ult to bring the enemy
into your team for an easy kill.
When
died that allowed your whole team to force a 4v5 at enemy base and finish them all off one-by-one until you grab inhibs and win.
That one play turned that game into a complete free win.
I can't go over every scenario (I think we'd be dead of old age by then) so for the moment just imagine similar situations to the
example.
Have a good day and Good luck in the rift...