To begin with, having supports giving steroids would be a very poor design decision. Things like Zeke's and Bloodboil are some of the least interesting support effects in the game. Janna's shield is the most interesting one, and that's because it isn't a straight steroid. Just as a support player, I have to say that I'd absolutely hate it if my role was just to buff a carry. I have much more interesting things I could be doing. There is a reason you never see any support player asking for support Nunu to be buffed back into viability. We do not look back fondly on our memories of winning games by spamming W on Caitlyn until she killed everything for us.
I would stop playing support entirely if my main job or even any part of my job became to provide a steroid to another champion. That is boring. I do not want to be a stat-stick for someone else. If you want to talk about unsatisfying gameplay, that is pretty much the definition of it right there.
Isn't the reliability and safety of range more than enough strength for a Marksman?
Nope. A basic facet of autoattacking is that it doesn't do much damage unless you have scaling mechanisms. Range and safety doesn't do much if you don't actually hurt. From a design standpoint, it is good that they have abilities and functions that support their key gameplay. There are carries without steroids or without strong ones like Lucian, Kalista, Graves, Sivir. That's fine. However, there is no reason why ever adc should have to follow that same pattern. There is no reason why they shouldn't be allowed to function off of autoattacking alone. Right now, there are multiple niches that carries can fill. Removing that diversity would be a very poor plan.
What melee carries do have is stronger steroids. No ranged carry except Vayne has a flat AD steroid, while tons of melee AD carries have them, and nothing a ranged carry has equals Trynd's 35% crit or Yi's E. There is a difference in potential offensive output.
The question is more why isn't there meaningful differences between Melee and Ranged?
There is a meaningful difference, and that difference is one of the reasons that double melee isn't going to work. The reason we don't see melee carry with support or double bruiser lanes isn't because ranged AD carries are inherently better than either of those classes. It is because they are inherently better for duo lanes than either of those classes. There are two main reasons. First, range is always going to be a massive advantage in duo lanes because it gives you a huge advantage for sustained trading. It also guarantees that you can farm if you're behind, which is important because a duo lane is extremely punishing if you ever fall behind. That's not due to steroids, or anything else. It's just range. You could (and I do) play Annie or Xerath or some mage and get just as much advantage. But, despite that, we don't see any double mage bot lanes. This is due to the second reason.
The second reason, and the more important one, is experience. Bruisers and mages need experience to function. So do melee AD carries. Ranged AD carries only need gold. This is huge. If an ADC and support are in a passive lane and get to level 6 without kills, the ADC can make do just fine with just a BF sword worth of gold. He can make something happen. A mage can not. If you have two level 6 mages when the solo lanes and jungle are level 7-9, those mages are screwed. Their potential impact is gutted. The same happens with bruisers.
If you want to see more diversity in lanes, you aren't going to get it by just nerfing AD carries. It won't change anything unless you nerf them enough that they aren't worth playing at all, and then you'll just have the next least exp-reliant class played with a support instead. You're not going to make double melee lanes happen just by nerfing range. If you really want it to happen, you have to mess with the way exp is assigned and you have to come up with new game systems that would reward different bot lane compositions. Just trying to nerf a class won't solve this problem.
