Code is definitely an asset, but it deprecates just like hardware does, for a variety of reasons like faults but also just the software and hardware moving on in the industry. The more code you have the more depreciation you are paying annually, which if you don't pay the system will eventually stop working and will in various parts fall into disrepair before that point.
The bit that is a challenge with the deprecation view of things is that if the code isn't sufficiently maintained (size outstrips developers) then the cost of fixing the problem increases with time disproportionately, its like you pay interest on the outstanding depreciation debt. So I understand why people use debt but its not quite right.