[mmsc]

Sure (and I'm not sure if that's actually how it works), but the border control also physically stops someone from leaving after they've overstayed. If someone has overstayed, they just go to the airport and hop on a plane with no consequences.

[zarzavat]

Some countries do fine overstayers, but UK is quite happy for people to leave with no penalty, it just doesn’t want them to come back again afterwards.

If someone overstays without a good reason then they are probably not entering the UK again for a long time.

The consequences are almost worse without the exit controls because overstayers will waste money on a flight only to get turned around at the border - assuming they don’t need a visa or ETA.