I think because for decades so many crimes were potential felonies that police and prosecutors used the misdemeanor/felony dividing line to systemize the allocation of resources--prosecute felonies, ignore misdemeanors. Most of these felonies were ultimately prosecuted as misdemeanors through plea arrangements, which shows another aspect of the system: heavy potential penalties made it easier to secure pleas, which reinforced reliance on felonies to filter prosecutions.

Now that penalties are actually fair, police and prosecutors need to change their habits. But they haven't, despite the fact that they'll openly admit they won't prosecute the exact same crimes they once did.

It's perverse and there's no excuse for it. Felonies may have made it easier to secure plea deals, but misdemeanors are easier to prosecute from a technical perspective[1] and because of the lower penalty easier to secure plea deals closer to the maximum sentence. It's an open question whether they're cheap enough, but nobody has yet even bothered to suggest it's not practical, let alone prove it or show that there aren't measures we could take to streamline enforcement.

[1] More safeguards in place to ensure fair trial when felonies are involved. Compare traffic court, where indigent aren't even guaranteed counsel. Indeed, counsel for indigent defendants is only required by the Federal constitution for potential sentences of 6 months or longer. First offense shoplifting kids could theoretically be swiftly prosecuted in the blink of an eye (especially given camera evidence), though in some states that may require changes in the law as defendant protections were often fashioned in the context of insane penalties for common offenses.