Why is cloud cost management so challenging?

Cloud costs are challenging to manage because to most people they are difficult to dissect and understand. If you are lucky to have a knowledgeable CIO / CTO that you can partner with during the negotiation of buy/renew, do it. The technology is advancing so fast making it difficult for the financial professionals to keep up with all the changes that undoubtedly impact pricing. 
It takes a village.

Vendors can’t estimate your costs very well. Often they simply say start with the free trial and see how much you use. However by then you’re typically invested.

Also, for those not in the US, cloud costs fluctuate with the US dollar which is another challenge.

Then there’s the whole issue of architecture. If people lift-and-shift their apps and on-prem VMs and treat the cloud like a data centre their costs will be excessive. Yet the alternative is to re-implement all your apps using the cloud provider’s service and who has time and budget for that?

Then there are the issues of contracted services vs. casual consumption pricing. Many companies know they can save money by locking in a commitment but lack the confidence in their forecasting to make that commitment.

Other options to save money include spot instances but these can be terminated at any time and thus the applications need to be written with that in mind - and again that comes down to architecture.

Plus you have developers and others who spin up machines … and then leave them there after they no longer need them.

Plus admins who don’t tag resources well - so others later say “resource X” but have no idea what it’s for or if it is in use so they don’t touch it and it’s simply burning money.

Ultimately, cloud cost management requires continual vigilance and it requires disciplined teams who are cost-conscious, and who have a structured approach to tagging, documenting, etc. and it also requires reworking your systems to the cloud way of thinking.

Unfortunately many companies lift-and-shift and don’t think about it further, and don’t understand why their bills are so high.