Also not everyone has easy access to an exchange. Others may prefer to avoid the KYC paperwork, or simply be willing to pay a premium for freshly mined coins.
Sure the client might be liable for some loss-of-earnings fees but this really should be better for everyone than this.
Others are FPGAs programmed to compute the hash for whatever cryptocurrency (e.g. BTC is `sha256(sha256(x))`); they can be re-programmed to do something else instead.
Others are ASIC (Application-Specific Integrated Circuits) based, physical chips designed to do nothing else but compute those hashes. So essentially 'no', but 'yes and they'd be very good at it' if you for some reason had another use for that particular hash function (lots and quickly).
That's also a progression of cost & performance, by the way. As the difficulty increases GPUs get slower at finding whatever coin, and people (pay a bit more and) move to FPGAs, and then again to dedicated ASIC hardware.