[Graham]

What Kevan said ...

Kids, Kids, Kids, Kids & Big Kids.

Off the shelf slot car sets such as Scalextric & Carerra are aimed at kids, sold in toy shops even .... for kids that have parents with big big budgets mind you, even if it is adults that play with them.

Therefore the marketing theory is, lets made them as appealing as possible. And, again, as Kevan said kids love lights and sounds. They also like building and destroying stuff.
So police cars that pop up from time to time are a given winner.
I am surprised though that the building block hotrods from a few years back didn't take off, I got a couple of them and that was all my young (then) kids played with.

For adult racers the likes of Slot.it, Fly, Pioneer, Policar, Racer, Revoslot etc do not have lights as they are more setup for full on racing with tuning ability.

Slot it supply lighting kits of course, I have known some National Le Mans events in the past requiring lights fitted for their 24 hour races.