Because the laws are different per country. Depending on the importance of cycling, riding, walking,… In Germany a signed cycleway is only to be used for cyclist, a bridleway is exclusive for horse riding,…
What you describe as highway=shared_use is highway=path in Germany. highway=path+bicycle=designated+foot=designated has about 277000 occurrences. If you are talking about ways in the woods, in Germany each state has it’s own laws. Some require 2m width of ways to allow cycling, some states have different rules.