Tradeoffs. silentbicycle's explanation is pretty good, but I want to call out the fact that you simply can not have an optimal source control system and an optimal binary blob management system. The two share a lot of similarities and there's a core that you could probably extract to use to build both, but when you're talking optimal systems, there are forces that are in conflict.
The problem is that if you aren't hip-deep in both systems, you often can't see the tradeoffs, or if someone explains them to you, you might say "But just do this and this and this and you're done!" Hopefully, you've had some experience of someone coming up to you and saying that about some system you've written, perhaps your boss, so you know how it just doesn't work that way, because it's never that easy. If you haven't had this experience, you probably won't understand this point until you have.
There are always tradeoffs.
Lately at my work, I've run into a series of issues as I get closer to optimal in some parts of the product I'm responsible for where I have to make a decision that will either please one third of my customer base, or two thirds of my customer base. Neither are wrong, doing both isn't feasible, and the losing customers call in and wonder why they can't have it their way, and there isn't an answer I can give that satisfies them... but nevertheless, I have to chose. (I don't want to give specific examples, but broad examples would include "case sensitivity" (either way you lose in some cases) or whether or not you give a particularly moderately important unavoidable error message; half your customers are annoyed it shows up and the other half would call in to complain that it doesn't.) You can't have it all.
The problem is that if you aren't hip-deep in both systems, you often can't see the tradeoffs, or if someone explains them to you, you might say "But just do this and this and this and you're done!" Hopefully, you've had some experience of someone coming up to you and saying that about some system you've written, perhaps your boss, so you know how it just doesn't work that way, because it's never that easy. If you haven't had this experience, you probably won't understand this point until you have.
There are always tradeoffs.
Lately at my work, I've run into a series of issues as I get closer to optimal in some parts of the product I'm responsible for where I have to make a decision that will either please one third of my customer base, or two thirds of my customer base. Neither are wrong, doing both isn't feasible, and the losing customers call in and wonder why they can't have it their way, and there isn't an answer I can give that satisfies them... but nevertheless, I have to chose. (I don't want to give specific examples, but broad examples would include "case sensitivity" (either way you lose in some cases) or whether or not you give a particularly moderately important unavoidable error message; half your customers are annoyed it shows up and the other half would call in to complain that it doesn't.) You can't have it all.