If your peak monthly cost for AWS services as a business is $100k, do you not think setting a $200k spending limit is reasonable?
Obviously, the system should provide ample time by warning in advance of reaching it (and could even offer suggestion to keep it at N times your peak from M months ago).
If as a business you set your spending limits so tight that you frequently run into them and it's not some unusual activity, the problem is not that spending limits are available :)
It is mostly about protecting from the unknown, likely unbounded attack on your infrastructure, where your spend might grow 100x: even if you can take $100k, you might not be able to take $10M in a month.
And if you use enough services, a global per-account spending limit can't distinguish between peak usage versus one service being abused.
Imagine you spend $100k on average each month, but spend around Christmas rises to $1m because of the specifics of your industry. With a global spending limit, you can't distinguish between $200k of general spend increases due to Black Friday versus $200k in fraudulent 2FA SMS to South Sudan.