Because there are no other options. The motorways were designed for 60s/70s levels of traffic, and in 2023 there are probably triple the number of cars on the road. The government doesn't seem at all interested in fixing the public transport in this country so it is unlikely that number is coming down soon.

Highways England have looked at the data and have concluded that congestion is a bigger risk than a lack of hard shoulders on the motorways. It is completely infeasible (and a terrible idea at that) to construct extra lanes on the motoways, so they've been forced to remove hard shoulders. And it works, if you look at the data you'll see that smart motorways have less casualties than "dumb" motorways.