Well there are plenty of 550s floating around out there used in this chambering with 23" bbls - quite a number of them. The "American" and others.
But if you're looking at the 550 FS, then yeah, it's always in 20", as are 99.8% of FS/mannlicher rifles (18 or 20").
Nothing wrong with this. There is no "should be". There is only differences of short/light/handy vs. some velocity loss tradeoff.
I'd love to have an FS in 6.5x55 and may get one yet.