Probably they think that they would not feel really at ease if they did. Personally we find that two paces and a half is our limit.
MORE THAN TWO.
Host. No, please don't sit there.
1st Guest. Oh yes, I much prefer it.
2nd Guest. Do let me.
Host. I can't have you sitting there.
1st Guest. I assure you I like being back to the driver.
Host. No, if anyone sits there, naturally it must be me.