shooters are a leftover from the bygone gays of gaming, the pre-3D era. i don't expect most folks to give them much mind who are not from that period.
back in the day, shooters were probably what made up a good portion of games released, they often had cool glitzy fast moving hi-tech graphics and metal/electro soundtracks, and they were all about jumping in and getting immediate gratification of gameplay. no generic leveling up, no slow progression - just put your quarter in, suddenly everyone's shooting at you from all sides of the screen and you better dodge and shoot back or it's the next guy's turn. Another big component, unlike many platformers of the day, were the proliferation of powerups in these games - you'd start out with a wimpy lil' pea-shooter but if you were good, you could upgrade your fighter into a competent killing machine dripping with lazery goodness and side ships and etc, filling the screen with all sorts of weird wonderful colorful gunfire.
also those games got HARD, where the gameplay was incredibly simplistic "move and shoot and dodge, wash rinse and repeat" but the goal was to just endure a little bit more than you did last time, see how long you could survive - an addictive test of one's reflexes.
yeah, so most people these days who are not from that generation will take one look at a thunderforce and an rtype or a musha, and be dismissive "what's the point!" But hey, "those of us who were there, remember why these games are fun" -- and why we still can get a charge out of them..