It's not about TDs, it's analytics. Take fantasy football as an example, when a stud running back gets hurt a majority of the time the backup can come right in and putup good numbers.A good RB will score more TDS than most WRs.
Like others have said, thry just aren't worth a first round pick very often anymore because the drop-off is much lower than other positions in regards to replacement level.
Take Dalvin Cook fro example, stud running back, when he gets hurt Mattison usually comes in and puts up similar numbers. But let's say Jefferson gets hurt, would you expect their 3rd or 4th receiver to start putting up Pro Bowl numbers as his replacement?