Because that’s not how dispatch works. Given the number of convertion methods defined, it’ll be extremely ambiguous in general which one should actually be called. (e.g. If you also have a convertion from NTuple{2,Float64} to Bar and have another method Bar(::Bar, ::Bar), which one do you want to call?)
It’s probably not impossible to come up with a set of rules to determine what is ambiguous and what’s not but given the “non-locality” of the information (i.e. it’s stored both in the method table of the method you are calling as well as the whole set of convertion rules, possibly chained) it’ll likely be very hard to predict and will certainly be extremely hard to implement dynamically with any reasonable performance.