I came a slightly unusual route, getting my PhD quite quickly and then spending five years freelancing, before being tempted back to academia. Oh, and I'm in the UK - it's a bit different in the USA. The base salary for a postdoc is not too exciting, but there are a few other things that make it attractive.
The first is that you get a lot more freedom than as a PhD student (or a junior employee in a corporate R&D lab). You start to be able to set your own research agenda. This depends a lot on institutions, but where I work there are a couple of projects with multi million dollar funding that are led by postdocs (a tenured faculty member has to be the name on the grant, but it's purely nominal). You may be able to supervise PhD students.
The second is the flexible working hours. I have a few hours a week when I actually need to be in the lab. The rest of the time, as long as I'm not blocking anyone else from getting things done, no one cares where I am (or what I'm doing, as long as some papers come out periodically).
The third is that I get to play with some very shiny toys. I'm typing this from a latest-generation MacBook Pro with all of the upgrades (2.6GHz CPU, 1TB SSD), which the lab bought for me yesterday, but that isn't too unusual for corporate side. Slightly more unusual is that when I started working here the only thing thing on my desk was an $8,000 FPGA board, which is just about to be replaced by a better one, and there's a big box of them if I need more than one (we're starting to play with boxes with 4 of the newer boards). The same thing extends to travel budgets. I've had a few months over the last year where I've claimed more in expenses than salary (which is less impressive when you remember the postdoc salary), and every time I go on a trip it's fairly common to tack some vacation time on. I don't really have to justify travel much beyond saying 'I'd like to visit this conference / university, it's probably sensible,' although part of that is the combination of funding rules that make it difficult to spend grant money on things that are not travel.
The fourth is that you are not limited to the working for the university. Most companies that want you to work full time expect you to work entirely for them. When I asked about consulting in my interview here, the reply was that of course they expected me to consult, how else would I stay up to date with trends in industry? You can add quite a lot (100% isn't too unusual) to your postdoc salary by consulting, and the flexible working hours make this very easy.
I interviewed at Google at the same time as I interviewed here. Google offered me quite a bit more money, but I don't regret making the choice I did. If you're thinking of a postdoc as a way of becoming more employable, then you're probably doing it wrong (unless you're aiming for a lectureship or a senior post in industrial R&D), but if you're looking on it as a way of being paid to have fun then it's a good deal. I'm basically doing now the things that I was doing in my spare time before, but now I have a lot more resources and I get paid for it. It sure beats working for a living...
Oh, and the $50K number you quote is close to the base salary for a postdoc here. It goes up to around $75K. I just checked a salary comparison site and apparently the postdoc salary is about the same as a software engineer would expect to be paid here, and about double the median salary.