Why ? Parliament and lawyers aren't capable of running safety reviews, nor do they have the will or (it seems) the ability or budget to provide an organisation that can do it.
So who then writes safety reviews ? Well, the same as for medicine : the company gets them written. Ideally by a somewhat neutral third party, but since they're paid by the guys who want things approved there's going to be problems here. Since the taxpayer won't pay for them, they are by necessity paid by the company wanting something reviewed. Those guys want these things approved and are paying the bills.
This is very much working as intended. Want a better system ? It will be much more expensive, as you'll effectively have to hire medicine experts away from companies like Monsanto + provide them with infrastructure.