I'm pretty sure it's because other industries wondered why they were having to spread such costs over 5 years while software firms were able to write them all down at once. It's not that I have a strong opinion about this either way (I'm not running or employed in a business where this matters), but that ultimately this is a philosophical argument. There isn't an objectively correct way to do this, how you view it is down to what your economic interests happen to be.

It's the other way around. Software used to be special, in that money the company spent to improve its internal processes by, say, buying a calculator had to be amortized, while money spent on developing software automation were not.