It's not that.
Back in the day, (take 1969 as an example) teams played their strongest lineups. We got to the final and on route there was that unbelievable game against Liverpool...Lochhead scored at Anfield and we soaked up relentless pressure. Shilton saved a penalty, it was blood and thunder! Fantastic football with all the teams really wanting to win it and none of this playing 10 reserves lark!
Compared to those days the F A Cup is a totally devalued competition.