This is one for my other half really (I just know the office stuff) but one reason is that a flexi hose is intended for an appliance that can be moved (ie for cleaning, access etc hence the need for a stability bracket or chain also.) whereas a fixed/built in hob does not need to be moved so a rigid installation is preferable and a chain/bracket is not required.
Most hobs are fitted into the worktop and the pipework is potentially accessible in the cupboard underneath. This could be cought or knocked by whatever is stored in said cupboard so rigid pipework is a safer bet than a bayonet fitting which is designed to pull off.
There are probably other reasons but, as I say, this is my partners area of knowledge, not mine (and he's not up yet).