The only reason is if it’s across Asok Montri ( 21) then you have to cross the main road while if it’s Centre Point or any closer to soi 19 it’s much easier.
I am on the car brigade to and from the airport. The rest of the time it’s BTS or the MRT or even the motorcycle or grab if I am lazy or too hot outside. Yes they have started charging extra for bags but to me the comfort of sitting all by myself and paying only 350 baht or around there is better for me than carry my luggage around.
That said I have used the BTS and Airport Link if I had to go to the airport for any reason other than arrive or departure.
The biggest problem is Thailand does have an integrated traffic management system the Asok -Phetchaburi junction feeds everything into that area and hence the gridlock.