it's the record/mix/produce/master part that most bands cannot do on their own very well... quite a few think they can... but the actual "good" albums done this way are few and far between. not talking about good music here, talking about good tracking, mixing, mastering.
nor the advertising, marketing, administrating, distributing.... bands usually suck at all of those as well.... and they SHOULD suck at all those things, because frankly bands should be putting their FULL efforts on writing and performing their music... not getting bogged down in every other aspect of the business... most of which takes just as long to get good at, IF the talent for them is there at all, as it took to get good at playing and writing music in the first place.
bands that "do it all" are usually just diluting themselves... watering down what they could otherwise turn out. you need a team behind you to really get somewhere. case and point: 99.9% of all bands that have ever "made it" in the history of recorded music, ever.