Your problem comes when you have to interact with anyone else. You could be stopped by one of the emergency services, you could breakdown and need roadside assistance, you could be involved in a RTC and need to exchange details to name but a few reasons.
If you're self isolating you must realise that you could be contagious so why put others at risk?
I agree but there's a big difference between Social Distancing and Self Isolation. The latter meaning you have have reason to believe you, or someone close to you, has come in contact with or is showing symptoms of the COVID-19 virus and as such must keep away from anyone else.
To believe you or someone close to you has come into contact with the virus to self isolate might be one reason, but like many others, people decided to self isolate to prevent anybody outside giving them the virus.
My wife at this moment in time will not go out shopping to supermarket for food because she is scared of catching anything so I do the food shopping once a week and also collect her medication from chemist -- we together go for a short daily walk observing the distancing rules at all times -- this weekend will be the test and I'm not looking forwards to media headlines telling me that people have been tearing up the rule book by travelling when told not to do so over the holiday
That chap who was pulled by the cops yesterday in Cumbria because he had a canoe on his car roof and he told the cops he was going to lake District for spot of canoeing with his friend -- he deffo had not got it