A number of reasons are given:
- at a time there was a limit to the length of the top tubes so they simply didn't get longer in standard materials, not sure I buy that one. But certainly many of us remember a less generous time when off the rack was not availabe to people of non-normal dimensions, or left handed, etc... To save money tube sets are made to a length that will accomodate most people whether there are any real limits here I don't know. It certainly is true one sometimes has to customize tubes to get various performance goals, even if they aren't specifically driven by rider size.
- There is sometimes a wheelbase expectation out there for a paticular use so the manufacturer's specs may claim all bikes of a certain type have the same wheel base, stay length, and head tube angle just to satisfy expectations. Obviously if you freeze the WB it get proportionately shorter in larger sizes.
- Wheel size is also an expextation, so as bikes get smaller there may still be presure to deliver them in 700c for racing or 26" for MTB which basically does the same thing as above.
- The wheelbase is sometimes optimized for shorter lengths for handling in various kinds of road racing, or for single track, so again as long as you can scrunch aboard there may be a minimum that is desired, this is sorta the underlying reason behind the expectation given in 2 above.
- People say what TM said above, though as a tall guy with short legs proportionatly, I tend to think it is only true in movies.