Cloud providers cut compute/storage costs every few years. You can get long term contracts with them that have heavy discounts for guaranteed spend. Switching compute instance types is pretty cheap, and switching compute platforms is relatively low cost when the time comes. Meaning that investors can generally expect your compute costs to fall YoY all else being equal.

Building out your own compute generally means adding new headcount to deal with colo things such as vender management, technical details such as networking, capes/opex management, and finally hardware selection. The odds that the hardware you select perfectly matches the compute you need in 2 years is low. The odds that a cloud provider has made a cheaper compute option in 2 years are moderate. The odds that your app needs something that’s available trivially in a cloud provider are high.