From PSU:

It is important to know that not all vegetable varieties are suitable for seed saving. If the variety you want to save is a hybrid, seeds from that plant will not produce genetically true fruits. Most likely, the plant will produce a fruit that resembles one of the plants used to create that hybrid. To avoid this, choose heirloom varieties, ones that have been around since grandma's time or earlier. [...] Heirlooms will produce offspring that are identical to the parent.

https://extension.psu.edu/saving-seeds-from-your-garden

The second generation (F2) is worse only in the case they are hybrid seeds (F1)

They're probably cloned.

Cloning plants is a thing people have been doing for a long time, and is one popular way to get good produce.
You also take a set of carefully selected plants and carefully breed them to get a plant that has the desired profile. It's the deliberate nature that results in a better food, not "not being a replant".

Pollinators don't care which plants they combine, so the natural way often produces a fruit that isn't as good as a food crop.
It's the real reason most farmers aren't actually super into reusing seed. It typically results in a lower quality yield.