You don't have any stats to back that up because the stats would not and do not back that up
Nobody is saying things weren't rougher and pitches tougher but that does not equate into being harder to score.
If a defender lunges in with a two-legged challenge for instance, if you skip away from it you're in space, leaving the defender on his ****. These days defenders stay on their feet more, making it harder for a forward to get past.
Defensive tactics have improved too, meaning it's way more difficult for teams to carve out a goal.
The playing skills of defenders in general is way better than in the past, they're more ball-playing footballers than knucklehead bully boys of yesteryear.
It's all very well for pundits & journalists to say it was 'tougher' in the olden days, in fact it may well have been, but that doesn't mean to say it was harder to score - and the stats back that up.