I thought the same too, to the point that maybe even lead acid batteries could work. Unfortunately, buses still care somewhat about weight--from a 2005 report: https://www.transit.dot.gov/sites/fta.dot.gov/files/Electric...

> Consequently, powering a full-size bus would require a battery pack that is unacceptably large and heavy, as well as too costly to make a battery-electric bus commercially competitive.

Of course, the energy density of lithium may change things!