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.