I'm not a fan of companies telling me what I want. I'm aware things take time, and the food we order from this place travels well and reheats well. Post a click-thru warning and let me decide for myself.

The restaurant probably doesn't want the bad press associated with other customers getting cold/melted/soggy/etc food. Also, the restaurant may want to limit the amount of takeaway, as the venue may be small to accommodate too many carriers coming and going. Also, the courier may not like the route, maybe it takes too long (due to distance, but also traffic and other road conditions) and it's not worthwhile for them. The point is that you are just one piece of the equation and those apps are not necessarily telling you what you want, but telling you what they want.

Shouldn't those concerns be addressed by the restaurant itself, rather through various delivery platforms? After all, there's no real difference between you going to pick up the food yourself and you hiring a contractor to do it.

There is a difference. People are far less likely to go themselves, so that may be manageable by the restaurant. The app brings a lot more orders in. The restaurant possibly solved the problem by telling the delivery platform. If they suddenly had a bunch of couriers showing up at their door at peak times their solution would likely be similar, by telling the customer that they live too far away so they won’t let them order takeaway or something else along those lines. But the restaurants probably prefer to deal with a single party that can handle that bit of customer satisfaction for them.

Why is the delivery service concerned about the food quality at all? Post office is not concerned about quality of products it ships and I don't see anyone expecting it to be.

The Postal service is a utility competing on efficiency and price. That's not what "tech unicorns" get billion dollar valuations to compete on. Tech is about building a "platform" to siphon off money for all eternity while doing little of the ongoing labor of delivering or producing the value. To build that platform you need a moat, because otherwise somebody will come in a compete on price, so you build brand loyalty by gate-keeping the quality of stuff on your platform. That way any competing platform will initially be primarily populated by all the stores too "bad" to be marketed with the incumbent.

It's quite simply not financially lucrative enough to run a utility for venture capitalists. In a more sane, less inflated economic climate, this might change.

Customers turn to the food delivery service first. The pressure to meet all customer expectations is huge given the competition.