the extra attack feature is a feature, spellcasters also get features that alter their damage, like twinspell. also martial charachters extra attacks are supposed to be balances for their lack of scaling damage skills and spells with higher damage or utility.
by level 5 wizard has access to scorching ray, haste, etc.
the level 5 wizard has two hastes, which is essentially extra attack for two encounters, if they choose, they can summon shadow blade and throw 2d8 daggers twice a turn.
1d8+4 averages to 8.5 damage 2d8+1(whatever your dex modifier on wiz is) averages to 10 damage.(and its a weapon so it crits)
mages are fine, they have tons of options, choices, features.
mages get big damage from spell use and basic damage from cantrips. casting 1 cantrip per turn is not how mages will realistically play except maybe eldritch blast warlocks. whereas attacking twice for a fighter or barbarian is basically peak play at level 5.
