At a slightly different tangent, & I completely get that some people could feed their kids better on a voucher, & others will think the offerings are shit/boring...
Before all this covid nonsense kicked off, I used to run a couple of school residentials a year, via a third party company. Theatre trip, workshop or other activity, night at a hotel, museum or theme park the next day, sort of thing. Not cheap, although obviously pupils receiving Pupil Premium were subsidised (& covered completely where needful).
There would generally be 2 days of coach travel, so packed lunches were included. They were very seldom eaten, as they were a bit crap & the kids tanked up on junk at service stations instead. One year I was a bit skint & took home & froze about 50 unwanted cheese'n'tomato rolls the kids didn't want when cleaning up the coach as we returned...my dc ate them as toasties for weeks.
Every year, I filled in the feedback & said 'yup, great trip, but your packed lunches are grim, sorry'. Every year I got the same response: we understand, but we provide a cheese'n'tomato sandwich, fruit, yogurt & a juice carton because we've tried to liven things up, & we can't realistically do it; if we offer choices, you have to deal with kids arguing about who wants what, which no one frankly needs halfway down the M1, so we do one vegetarian, halal, fits all except food allergies options & that is what suits our business model.
It's the same set up. A bland, 'edible for just about everyone' box is the easiest option, & that keeps it cheap & practical.
Yes, personally, I'd definitely prefer fifteen quid to spend on fresh food & pulses, tbh. But as OP pointed out, not everyone can cook. If you're trying to benefit as many people as possible, most adults can make a cheese sandwich & most dc can eat one.