"Ancient" in Chinese stuff is usually a bad translation that originally means something more along the lines of "Primordial" or "Original". In the case of cultivators or superheroes, the original first dudes usually are the most super-powerful ones because they had to forge their own paths without any prior experience or guidance, so if they have normal strength then they already must have had ridiculous talent compared to the normal "modern" heroes/cultivators.
It's also because for whatever reason, the powerful beings have a tendency to become pacifist (prefer peace over thousands of years of fighting), and end up hiding in the background.
The other reason why old dudes are usually super-powerful are just time to accumulate their strength.