The main issue is cost.
Custom made easily 2x, 3x or even more costly than pre-made.

Premade- you can see the quality and feel it.
Custom - You won't able to know their workmanship quality until it is done, by that time, it may be too late. So it is "riskier" than premade. Also, you need to do some 'homework' yourself, so that the contractor won't be using cheap quality material.

Premade - as time goes, broken or whenever don't like it, just discard and buy new one.
Custom - Sometimes dismantle also big headache.

The advantage of custom made is you can "design" whatever you like, and utilise fully the space, but at the expense of cost and risk of unknown workmanship quality.