Not to mention a bin of 100 plastic army men weighs a helluvalot less than a bin of 100 pewter army men.
Furthermore, paint binds a lot better to plastic than metal. Even if you properly primed your models and sealed them with lacquer, handling metal models would eventually cause the paint to rub off of raised details and your model would end up looking like shit eventually. You were better off not sealing since at least then you could do touch ups, but it was still bad. Paint primer contains acetone which makes it binds to plastic, and the acrylic paint binds to the primer, which means your paint jobs will be much more durable on a plastic model than they ever would be on a metal one. Hell, even resin models bind better with primer than metal.
Another great advantage plastic models have is customization. Metal models typically come in only one or two pieces. With plastic kits they give you a ton of different options for different limbs and poses that you can customized to make varied battle formations and soldiers with different heads that look more natural, rather than a bunch of clones off of an assembly line. You can fudge the limbs around when the cement starts to set up too, allowing for even more posing.
And back to resin, it's somewhere between metal and plastic as far as customizing goes. Resin minis generally don't come with spare bits and are usually only one or two pieces, but with a jewler's saw (or even an exacto knife if you're careful), you can often times cut them up and make changes to the pose with a little pinning and greenstuff. This was possible on metal models too, technically, but was far more difficult due to how hard and dense the metal was. Using a tiny hobby drill on plastic and resin is super easy, but drilling holes in metal was an agonizingly slow process that I do not miss, not to mention all the fuss of jamming a piece of paperclip into a tiny hole in a piece of metal.