FACT! Last I looked the modern ratchet box wrench mechanisms were only good to about 250ftlbs or so. 21mm is about the useful limit (with a safety factor).
By "modern" you mean Gearwrench style? Even GW makes ratcheting wrenches up to at least 50mm. Even the bigger sizes are making it mainstream. Here's the front page of Canadian Tire (sort of like Sears) with a cheap Gearwrench-made set that goes up to 24mm and 1 1/8". (and before you say it - yes - no jack stands from a retailer that sells them

... tisk, tisk...)
I know what you mean though - I like my Mac 6-point flex head ratchets but can't get them over 19mm. I'm sure if there was a business case for making them, it would already be done more widely, but ratcheting wrenches are not really a replacement for the regular variety, which are already a specialty/industrial item as you move into large sizes. Demand is much smaller, the supply to meet that demand is much more limited and, as a result, prices are WAY higher. Once you're spending that kind of money there are lots of options depending on what you need.
Gedore was already mentioned for their newer style wrenches, but Carl Walter and Gedore also make much larger versions which I believe are 'friction' style ratchets. These go from 6mm to 115mm+. I've seen the smaller ones with much longer handles than is listed below as well.
Samstag sells both the
Walter and
Gedore versions (scroll down to find the series - if you need to check the price it's too high!

).