That's due to a trade-off of requirements, to maximise free and fair representation.
Lack of postal votes makes the ballot less fair and representative, because it affects people with systematic bias in relevant sub-populations (wealth, working conditions, age, health, etc) and areas.
In areas where intimidation can happen... for going to the polling station. (All the intimidator has to do is post menacing guards, soldiers, etc. outside the station or on the routes to it. There are plenty of news reports of this happening in some countries. I'm not aware of this in the UK though.)
So the question is whether you get a more fair and representative vote outcome by allowing postal voting, or by disallowing it.
The balance of trade-offs has led to UK policy allowing postal votes, encouraging each individual to fill out and seal their vote in private, and use statistical and other investigation methods to look for signs of fraud, while maintaining a high standard of secret ballots when voting in person.
That might not be the balance that works best in other countries. In the UK it is said by the Electoral Commission that there is no evidence that postal voting has changed electoral outcomes to date, but some attempts at large scale fraud were discovered and prevented.