Aren't they basically made up of the same stuffs (quarks) in almost the same configuration?
Because quarks come in 3 colors, there is no decay path from a proton to any number of particles whose sum mass is less than that of a proton.
There exist no particles that consist of 2 or 1 quarks (pions consist of a quark and an anti-quark). This leaves only the Δ++ as a reasonable choice, but that works out to be heavier due to the Pauli exclusion principle. It decays into the proton rather than the other way around.
There is hypothetical proton decay in the GUT theories; however this has never been observed. (Something happened a few years ago I can't trace and all the old reports giving proton decay time < bismuth decay time are wrong, and the report of a detector pile big enough to see one proton decay per year is preposterous as 10^8 tons monitored sounds preposterous.)