Because they do?
Unless you're able/willing to spend north of £40k, the only options I'm aware of with a decent (e.g. > 400L) boot are limited to either the MG5/ZS, Kia Niro, or one of the hideous Rifter/eBerlingo/Vauxhall Combi type van/car monstrosities. Of those, only the MGs and Kia have what I would consider a reasonable range for a family car.
It is getting better though - there are a few new options coming to the market which have more space, but that's only really in the last year or 2.
I'm old enough to remember a time before SUVs were popular, and if you wanted a practical car an estate was the go to. Maybe I'm old fashioned, or maybe I just realise that the (relatively) long, low and more aerodynamic shape of an estate is more practical, more economical, and offers a better drive* than a tall, boxy SUV.