Aws is useful if you have uneven loads. why pay for the number of servers you need for christmas the rest of the year? But if your load is more even it doesn't make as much sense.
In the UK we have a huge charity fundraising event called Red Nose Day and the public can donate online (or telephone if they want to speak to a volunteer).
The website probably sees 90% of their traffic on the day itself - millions of users - and the remaining 10% tailing off a few days later. Then nothing.
The elasticity of the cloud allows the charity to massively scale their compute power for ONE day, then reduce it for a few days, and drop back down to a skeleton infrastructure until the next event - in a few years time.
(FWIW I have no clue if Red Nose Day ever uses the cloud but it's a great example of a business case requiring temporary high capacity compute to minimise costs)
Everyone scales up around Christmas then scales down afterwards. What do THEY do with all the unneeded CPU-seconds for the rest of the year?
Also, every unused resource goes into the spot market. They just have a bigger spot market during the year.
And lastly, that's why they charge a premium. Because they amortize the cost of spare hardware across all their customers.
Plus bidding on spot-instances used to be far less gamed so if you had infrequent batch jobs (just an extreme version of low-duty-cycle loading), there was nothing cheaper and easier.
I've been out of that "game" for a bit, but Google Compute used to have the cheapest bulk-compute instance pricing if all you needed was a big burst of CPU.
It's all changed if you're running ML workloads though.