I don't care what people think, but restaurants that don't do takeaway simply don't have containers to take food home. I've been to 10 restaurants this month, and none did takeaway.

Besides, a half-eaten reheated meal doesn't sound very appetizing. The sauce will have congealed or soaked up by the food, the pastry gotten soggy, etc. etc.

A handful of reasons, I think.

Fatigue with the particular dish (I don't want to eat the same thing within a day or two anyway), a mostly irrational "ick" hygiene factor, not wanting to carry food around (it's less common to be parked close to where you're eating in the UK), a lingering (though lessened) elitist attitude to the practice, and a fear of it being considered an unusual request.

Different cultures engage in slightly odd behaviors (such as the prevalence of stick shift cars in the UK) often because people are, en masse, afraid of being unusual and making a change. If restaurants made taking stuff home an obvious and acceptable policy, I'd probably deliberately eat only half the meal and do it from time to time.