It's a thin-margin business – the majority of restaurants ultimately fail – rents are expensive, they need high levels of staffing etc. The Jamie Oliver chain wasn't the only one to fail, they've all struggled (and few of the remaining ones will survive COVID). You could cook everything diligently from scratch, but who's going to pay for it, when you can pop down the street to Zizzi or whatever and get an adequate meal for less – you can get meals in pubs for under a tenner, I'm sure they're microwaved, but it's food. I get the consistency thing, though it's a tad disappointing. That said, I've often eaten in the Cote places, which I'm sure is much the same model – if there's a group of you, you can mostly be sure everyone will be fed, and the food is perfectly acceptable. The new infestation of Ivy restaurants, I imagine is run on a similar basis.
And I don't imagine for a moment that Jamie Oliver was involved in the day-to-day management of the chain. I think I only ever ate in the one in Gatwick, which I expect was a franchise anyway (most of the 'restaurants' at airports are run by the same company, they just put different names atop the door, and it's not like they have to try too hard, they genuinely do have a captive audience).