Not an expert, but have worked on phones before. My guess: much more aspects of mobile phones are patented or secret, parts aren’t available to small purchasers

Not an expert, but have worked on phones before. My guess: much more aspects of mobile phones are patented or secret, parts aren’t available to small purchasers, weak OSS culture around PCB design at the level of complexity a mobile phone achieves, lack of support from carriers, general complexity and variety of disciplines required to successfully construct a phone. Remember that there’s also not really that many open source computers either. The difference is mostly that computers are designed to be general purpose OS booters, whereas phones are locked down consumption slabs.