All unpaired hands have the same chance of hitting the flop. They'll hit about 1/3 of the time and miss 2/3 of the time.
Yes, a pair heads-up is very strong, but hand selection is still necessary because of what cards can flop against cards you're holding.
In 67s vs AKs, both have the same 33% chance of pairing of their hole cards on the flop. However, 67s is only flopping a top pair hand ~4% of the time, mid pair ~15% of the time, and bottom pair ~10% of the time. So, 75% of the time, an overcard(s) will be flopping against mid or bottom pair. AKs will hit top pair ~29% of the time and will never be mid or bottom pair. Both hands have the same 67% chance of missing entirely, but A high is winning against 7 high 100% of the time.
It's also a lot harder for a hand like 67s to continue on most flops even when they than it is when AKs misses. On a flop of 6-9-Q is probably getting check-folded by a 67 hand. And even if they bet, get called, it's hard to continue on the turn. However, a hand like AKs has no problem c-betting/calling a bet on this type of flop.