For one thing, gen four was over a decade ago and times change. There are very few Pokemon who can claim to be ou in every single generation. Starmie just isn't one of them. Also, Starmie doesn't counter Landorus Therian. It only checks it. It just about dies to any attack Lando throws out if it recklessly comes in. Cinderace is also faster than it and can change type at will, effectively negating most of Starmie's damage output and soccer bunny is now banned from ou by the way
To put it simply, Starmie fell off because of power creep. With each generation, new threats come that are either faster, stronger or bulkier, forcing certain mons to adapt or be kicked out. Starmie's usage basically fell out since gen seven as the threats were too much with monster walls like Celesteela and the offensive threat to end all offensive threats, Ash Greninja, joined the game. Starmie just couldn't keep up
This was made a lot worse as generation eight introduced every water type's greatest nightmare, Rillaboom and its priority grassy glide. Starmie's high speed also isn't that special anymore with speedsters like Tornadus Therian, Zeraora and especially Dragapult roaming ou. It's rapid spinning isn't even that needed anymore as defog and boots are everywhere and most players would prefer to use a mon that matches up well against the rest of the metagame, even if that means having to concede your own hazards. It's also incredibly fragile especially on the physical side, meaning that ou's abundance of you turns severely hamper its viability
If you check the list of all ou viable Pokemon, you will see that Starmie doesn't matchup well against many of them. It gets destroyed by the aforementioned speedsters, walled by the Slow twins and Galarian Slowking and perhaps more importantly, absolutely hates knock off, which is everywhere in the game. Ou also has some sort of emphasis on switch moves and its vulnerability to the most common of them only makes it a bad pick
In short, it doesn't match up well against the rest of the ou meta game
