Legendary = more powerful
Blaziken is stronger than all three of the legendaries I listed, even without its mega. In fact, Moltres and Entei both have only a 580 base stat total, which is lower than the pseudo-legends. The titans are also legendaries, yet Regigigas' ability utterly cripples it, and its three disciples aren't much better. If you want to get really technical, Phione is also considered a legendary by some (not me), yet is utterly unusable and doesn't need to exist at all.
Yeah well all eeveelutions can’t go further so even if we went to second evolution
They're still final-stage Pokemon. Arcanine is a stage-two and is still stronger the Flareon, and they debuted in the same generations.
Flareon - PU
Vaporeon - NU
Jolteon - RU
Umbreon - RU
Espeon - RU
Leafeon - PU
Glaceon - PU
Sylveon - UU
None of the Eeveelutions are exceptionally good at what they do: Eeveee is a jack of all trades, master of none, and exists as a gimmick Pokemon that only continues to be expanded upon because of its undeserved popularity. It's the same reason Pikachu is expanded on as a Pokemon in every generation.