You can buy a raspberry pi zero for $5, which is more capable than any common graphing calculator. Why aren't Android/Linux based calculators more common? The hardware could be capable of so much more than any offering from Ti. There would be security issues but phone manufacturers like apple or Samsung can lock down their phones to a reasonable degree, so I don't think that jailbreaking would be too widespread. What are your thoughts on this?

You would have to do all the soldering and write an os from scratch if you made one yourself. You would also need all the parts, and no matter what if you put it together, someone will always be able to find a work around to break it apart. And with phone calculators being unable to be used, it is due to the fact that outside communication could be used on tests and the onternet. The reason people use these things is beacuse they normally have to for school, and then discover some kind of passion them and take them up on a hobby.

A big part of it is acceptance by (a) standardized testing bodies and (b) teachers. Both are slow and very set in their ways, the former especially through some combination of general standards organizational inertia and lobbying by/relationships with existing calculator companies. The technical problems with creating a new graphing calculator with an EOS and graphing capabilities are relatively minor compared to convincing the College Board and the ACT to allow it, and teachers to be willing to learn the keystrokes to use a new calculator. For the latter, my experience is that many teachers aren't hugely technologically savvy, and if they learn to teach with one calculator (especially if textbooks or reference material show how to teach with that one calculator), they'll insist students continue to use that one even if a "better" (or cheaper) calculator is available. TI has been very successful, for example, in providing professional development to teachers to instruct them on how to teach with TI calculators.

Plus currently calculators suit classwork perfectly well - there is no real need for any additional processing power for the classes that calcs like the TI-84+ series are designed for.

That being said there are ARM based calculators from both TI (Nspire series) and competitors (such as Numworks) for more advanced uses.