They're quite expensive for a start and standard HSR does it's job just fine.
Japan is the only country that's building actual Maglev lines. It's feasible in Japan due to popularity of rail and distance between the endpoints makes it worth it.
China has Maglev tech and also some demo Maglev lines. But they are committed to standard rail because it's cheaper to build using a standardised process and works good enough on large distance travel required in China.
In the US, it's nearly impossible because Petroleum companies and such hate the idea of cheap and efficient transport and just bribe the politicians to be against it.
TGV in France is super smooth. Maybe not quite as smooth but still smooth enough that you can have a tall bottle or glass on a table without fear it’ll fall over.
Then you’ve got a couple of places in Europe that hits 300 km/h, or near enough: Köln-Frankfurt and parts of the München-Berlin in Germany, Barcelona-Madrid and the Eurostar. All of these are super smooth.
The rest is just “high speed” marketing, sometimes done on tilting trains that’ll hit 250 km/h. The ones I’ve tried are not super smooth. Parts of the “higher speed” tilting trains in the U.K. are downright uncomfortable and can leave you travel sick at times.