Although the cost is relatively high for a glorified calculator, not everyone is technically minded or want spend time to assemble, test and troubleshoot the most important key of their lives. The user experience and convenience of buying a reliable signer can trump cost, especially if it gives peace if mind.
Discussion
It's not just about the cost, I think we have a user experience that I consistently hear is at least on par, if not a step ahead of most other wallets. Forgive me for asking, but since you commented on the time required to "test and troubleshoot" -- have you used a SeedSigner?