Under the logic of this rule, a firm making a software product could invest money in software development, produce the necessary software, then get income from the software without further investment in software development.
In practice this is never the case; revenue gained from software needs to be met with further software development to maintain, update, secure, etc. the software. Firms that invest in capital often have large startup costs that go down as the firm becomes fully capitalized. Software development costs seldom go down, but instead expand with the firm's success. This is counter-evidence to the idea software is capital.
The software produced is also of unclear value and is not fungible. If a firm buys manufacturing equipment and the enterprise is unsuccessful they can sell the equipment. In the case of an unsuccessful enterprise the software almost always is of zero resulting value. Given these rules if a firm invests money in software development, makes some revenue, but ultimately doesn't create a sustainable enterprise, 100% (or more!) of the profits could go to taxes with no ability to recoup the overtaxing when the firm is dissolved.
Additionally, a firm buying durable goods will be able to buy those goods on credit, using the durable good as collateral. The tax laws encourage this process and amortization makes sense. Software cannot be produced on credit, and in practice can never be used as collateral on a loan.