Originally posted by InfamousModders
They didn't do it because they were lazy and/or they believed that it would be hacked regardless or that they didn't have the ability to do so.
If they can code the game and its network, then it wouldn't be hard to code such a thing. Serversided stuff have been very common since the 1990s.
As for Destiny, there are ways to hack currency / coins / etc. By "hack", I don't mean it literally, but exploits (like duping or tricking the servers to accept your modified changes somehow) should work because nothing is 100% invulnerable. You don't get to see exploits in public that easy because they are easy to get patched by the devs; they are usually very private.