I don't disagree with you, but mbed TLS and other similar libraries partly exist to solve the need for high-quality crypto/TLS in very small operating environme

I don't disagree with you, but mbed TLS and other similar libraries partly exist to solve the need for high-quality crypto/TLS in very small operating environments, where it may not be possible or easy to use OpenSSL. They are designed to be highly modular and lightweight, and easy to integrate custom crypto acceleration hardware. I don't know enough about OpenSSL to say whether it's easy to work with in these environments, but this is why these idiosyncratic libraries exist, and it may not be as simple as 'just use OpenSSL' in all cases, especially for the non-TLS crypto parts.