1) Much more often than not, the ready-made building blocks are crap. Your software will reflect that crappiness, and your life will consist not of writing software, but of maintaining and massaging crap.
2) Even if your ready-made building blocks are high-quality, they limit what kind of software you can write. Ideally, you imagine what you want your software to do, and then you write software to do it. You create the building blocks in that process. If you are only using ready-made building blocks, there is a lot of great software you just cannot write. And I think we all see that, because there isn't much great software around.
These problems might not be a concern for you. If you write software that is pretty much just more of the same, 2) does not apply to you. And while 1) is still problematic, it will be just manageable because you use these building blocks mostly on their happy path.
But I have bad news for you. This kind of software will soon be written not by you, but by an AI.