They can't. But in that sector, good service is not the only pull factor - for some of them, service is not a pull factor at all. The low fares' biggest pull factor is exactly that - (perceived) low fares. That's why they work so hard to market that feature of their proposition. If they lose their pax to competitors, it'll be more likely because the price is wrong, not because the frills are absent.
But fares is also a variable for legacy Economy.
How come that low-costs can offer low fares and still make a profit, whereas the legacies are unable to offer cheap Economy tickets?