The difference is that an existing IT system is real and can be purchased for X amount. A potential future one cannot - and while devs are able to estimate (poorly) how long it will take others cannot and there is a real human bias - in fact one could say the future system is the fantasy
Purchases are good in the short term. You can evaluate them and pick the most suitable. Once you have settled with one product and know what you want it may make sense to work on optimizing your business with a custom fit solution.
