It's easier to hire someone who knows aws than to train someone on your custom thing. I don't really agree, but that's mostly the reasoning.

Amazon has a government cloud offering https://aws.amazon.com/govcloud-us/

Another question could be : which developed country is not yet using the popular AWS already and why ?

For example : https://press.aboutamazon.com/2023/10/amazon-web-services-to-launch-aws-european-sovereign-cloud

Customers, AWS Partners, and regulators welcoming the new AWS European Sovereign Cloud include the German Federal Office for Information Security (BSI), German Federal Ministry of the Interior and Community (BMI), German Federal Ministry for Digital and Transport, Finland Ministry of Finance, National Cyber and Information Security Agency (NÚKIB) in the Czech Republic, National Cyber Security Directorate of Romania, SAP, Dedalus, Deutsche Telekom, O2 Telefónica in Germany, Heidelberger Druckmaschinen AG, Raisin, Scalable Capital, de Volksbank, Telia Company, Accenture, AlmavivA, Deloitte, Eviden, Materna, and msg group

In Portuguese: https://www.serpro.gov.br/menu/noticias/noticias-2023/serpro-lanca-nuvem-de-governo

Brazillian government launched its own cloud service to support the government agencies, everything stored and administer in Brazilian territory, making it independent from private companies and international governments.

Cloud presents several advantages,and GovCloud is a thing.

Like, Amazon has SCIF cloud offerings. These leaks were cuz some dumbass contractor exposed a repo to the internet

I expect the same reasons they're mostly all using Microsoft Office, Windows, and Active Directory. Because it's cheaper than doing it yourself.

This comment makes it clear you've never worked in government IT.