The problem is that most of the hardware is not open. It is hard to get fully open software running in proprietary environments like mobile phones and networks. The fairphone might be worth a look though: https://www.fairphone.com/en/
Can someone enlighten me? I do understand what blobs are, but I guess I'm not clear on why they're needed.
What you need to understand is that your mobile phone is not a single computer. There are at least three computers inside your phone that have their own CPU, memory and ability to run code.
First, there is your SIM chip. Your SIM chip is a fully functional, standalone computer with its own CPU and memory. Your SIM card can (and does) run arbitrary java applets that can be forced onto it by your carrier without your knowledge. Yes, that is indeed as terrible and horrifying as it sounds.
Second, there is the baseband processor which handles all of the tricky real-time radio comms with the cell towers and which could not be preempted by your browser or other apps you would be running on the AP (see below). The baseband processor is not controlled by you, can be controlled by your carrier, and in many cases has DMA control over the memory in your application processor (what you think of as your "phone"). Yes, you read that right - many, many mobile phones can have their physical memory directly manipulated by a baseband processor that they do not control. Remember that next time some cute "secure" messaging/encryption/marlinspikey/secretive communications app gets released.
Finally, there is the application processor which is the "actual" CPU and memory that you think of as "my phone" which runs things like the uber app and chrome and facebook.
We are sort of, kind of, getting closer to having a free/open stack on the application processor only. There are two other very powerful agents in your telephone that we have made almost zero progress in opening up. There are very powerful financial interests that stand in the way of opening those up.
Sounds pretty logical though. As @rsync said, there are very powerful financial interests behind the hardware backdoors in phones.
As natch said, lack of documentation. This lack of documentation prevents others from developing their own drivers etc.
>why so many manufacturers do not make "open" hardware
Multiple reasons. The first four of which I am confident that often play a large role, and the two last of which are more speculation.
1. Companies might feel that open hardware would put them at a disadvantage because others could copy them.
2. Company might have licensed portions of the hardware from other companies and therefore must adhere to agreements about not sharing information about the licensed hardware with others.
3. Maintaining an open project is a lot of work for a company. Everything from ensuring that the whole process is repeatable, to having a team of lawyers approve everything for release.
4. There is no incentive. Users buy their phones either way, even if everything is closed and proprietary.
5. Regulations regarding wireless communications hardware might apply?
6. Company might be using software tools in the hardware development process that they can't share freely so even if they did share the files others still wouldn't be able to use the files for anything?