I think in terms of US adoption the cultural landscape is so different that it's hard to grasp.
If you're unexpectedly pregnant in the UK, you can access abortion fairly easily, and for free. Or, if you decide to go ahead with the pregnancy, there's a welfare safely net which, while not at all generous, leaves you not completely without means. And, the NHS picks up the cost of your antenatal care, whether it's straightforward or complex. You get to make that decision based largely on your feelings and personal situation - it may be hard, but neither path is impossible.
Then head across the pond There are many parts of the US where abortions are hard (if not impossible) to get. If you can get one, you'll need to do it quickly and there will likely be a cost to you - for the procedure, and perhaps for travel to where you can get it, and perhaps for a stay there in order to meet mandatory waiting times - and you may not have that money or that time. The religious sigma is also much more substantial and so you just may not feel able to go ahead with it anyway. It, you didn't even know you were pregnant until past the time your state allowed abortions anyway.
So, you're having a baby - but there are medical bills mounting up and the insurance system isn't your friend, and then maybe you live somewhere where the welfare assistance available to single parents is deliberately awful to 'send a message' and deter you from making the choices you didn't really have in the first place... It's all too easy to end up in a situation where both paths are impossible.
Which leaves adoption. Where you can get help to meet those expenses, and you can know that the baby is going to a family where they can give it all the things you can't; you can actually choose who and you can specify whether or not you want to remain in contact... It's a way out. I can completely see why people take it.
It's that highly problematic? Yep. But it's not really the adoption system that's the problem; it's all the other systems around it. I know people on here are very anti-surrogacy (I don't entirely agree with all the arguments against, but I recognise them) but it is a situation where there's a choice about whether to become involved. In terms of adoption in the US, on the other hand, that choice is somewhat illusory since it looks very much as though it may be the only viable option for many who find themselves pregnant unexpectedly.