Building a business on blank cheques and accidental spends is shady. It's also a large barrier to adoption. The more times devs see reports like, "I tried [random 20-minute tutorial] and woke up to a bill for my life's savings and luckily support waived the fee this one time but next time they're coming for my house", the less they'll want to explore your offerings.
Even having the option of a hard spend limit would be hazardous, because accounting teams might push the use of such tools, and thereby risk data loss incidents when problems happen.
Hard spend limits might make sense for indie / SME focused cloud vendors though.
In the olden days if we spotted a customer ringing up a colossal bill, we would tell them. These huge Amazon bills are fast but still multiple days. They can trivially use rolling-projection windows to know when an account is having a massive spike.
They could use this foresight to call the customer, ensure they're informed, give them the choice about how to continue. This isn't atomic rocket surgery.
"Oh but profit" isn't an argument. They are thousands of dollars up before a problem occurs. The only money they lose is money gained through customer accident. Much of it forgiven to customers who cannot afford it. It's not happily spent. They can do better business.
You can't even setup Cloudwatch alerts on your actual spend, the only metric available to you is "EstimatedCharges".
Again, Amazon isn't stepping up to protect users from themselves. They could do a lot more.