Most don’t bother (perhaps not realising there is supposed to be a seal in the gap).
“Some have opted to use pipe insulation and when installed neatly does look better than the typical foam that is sold as an original replacement part. Adhesives are commonly used to hold the material in place, but stories continue about how the foam is sucked into the fan and shredded”
Read how to make a permanent type of seal at ……. https://www.britishcarforum.com/bobmuenchausen/42187.html
More about the original seal that you rarely see fitted at …… https://mgbtips.wordpress.com/2019/08/03/radiator-fan-seal-and-overflow-fasteners/