Is there a reason why mages have manabars nowadays?

Good mages are smart with their mana usage.

Mana in general has become an increasingly less relevant resource over time. No modern champion is designed with mana issues, and the older champions that are tend to feel clunkier and less fun to play as a result. Additionally, there are currently a few runes that provide a significant boost to mana and mana sustain, which makes mana easier to manage than ever. Personally, I think this is okay so long as champions retain their interactivity, as I'd much rather have an interactive mage than one whose lack of counterplay is counterbalanced only by dysfunctional mana consumption. When a champion isn't interactive and has no mana issues, that's when they start to become much less pleasant to deal with.

Another thing to consider is how much the landscape of league has changed over the years.

Mages need to be basically manaless to be competitive with the adc and bruiser roles.

umm yes and no... if u get an then yeah you are pretty much never going to run out of mana EVER, but mana regen is pretty bad wihout it.

its just the decent ONLY mana option now even if u dont scale