Collecting money is an important part of a business, but also not trivial. It therefore makes sense that people would outsource this. If you get big enough, you probably don't want a middleman in the way; this is why companies like Blizzard have their own store/launcher app. For everyone else, whatever they take is cheaper than having their own army of software engineers, lawyers, billing phone support, etc. In a world with Steam, as a game company, you don't have to worry about collecting money (and the associated customer support) and distribution. You can focus on your game. That is valuable, which is why people pay for it.
I work at an ISP. Billing is a constant influx of "interesting" requests that I would love to outsource so our software engineering team could focus on stuff specific to our business. But we never found a service that could do it better than our homegrown system.
Every week there are several one-off issues like "I overpaid because I didn't see the service credit" or "I would like to pay for the next 3 months in advance" or "my accounts payable department sent the wrong check, can you shred it and we'll pay with credit card". These things have to be handled manually. Not to mention people calling to change their payment method because they don't remember their password to the website, or just want to chat. All that is a drain on actually running the business, but unavoidable. You can't just say "nah we won't shred your check", so someone has to go find it and deal with it. While they're doing that, they're not making our product more innovative or reliable. They're running a check through a shredder. Anyone could do that, but unless you have 8 hours of check shredding to do a day, you are spending CFO salary on the task instead of check shredder salary on it. Therein lies the problem.
If this could all be outsourced, I'd pay a lot of money for it. So would any business. Billing is something you have to do if you want to collect money, but it's a time sink. Game developers don't have to worry about this, and generally seem OK with that. I don't blame them. They are lucky Steam exists.