What's the deal with fast food delivery services?

Likely due to costs of having deliverymen on staff and the difficulties to keep their food hot prevented them from doing delivery services.

Even now, the restaurants don't have their own delivery persons. They all have made partnerships with delivery companies like DoorDash and Uber Eats to make the deliveries.
https://www.businessinsider.com/mcdonalds-delivery-growth-2018-7

Those delivery companies emerged recently from the growing sharing economy, and rely heavily on smart phones and apps, which did not and could not exist in the 80s and 90s.

We also didn't have much of a gig economy then.

There's a Texas hamburger chain, Whataburger, that in the late 80's started delivering orders, but now they're hooked up with third party delivery services.

That's what I was referring to. You forgot GrubHub.

I didn't want to list every delivery company. I figured I made my point.

Yes you did.

The idea of fast food franchises such as McDonald's, Burger King, Wendy's and KFC using delivery services seemed a bit weird at first and took some getting used to.

I often order pizza online.

Doesn't an efficient delivery service require the modern internet?

Chinese food and Pizza are a little less time sensitive.... if Chinese food or pizza sit in the car for 30 minutes in insulated bags, you won't complain too much. I think that is why these were your delivery options in the 1980s.

You don't want to eat 30 minute old french fries, or a hamburger made 30 minutes ago.

What stopped fast food franchises from instilling delivery services back in the late 20th century? They could have had toll-free delivery services.

They could have advertised them on TV and notices about the restaurants.

The advanced on-demand food delivery business models have led to food ordering, delivery, payments, and everything online. People have switched to online food delivery because it is much safer, convenient, and saves them from the effort of stepping out.