[flyinglizard]

1. There are many, many more Western countries other than the US.

2. Even if they develop their own tools and research their exploits, using NSO provides a layer of plausible deniability and hiding behind someone else's fingerprint (think about the command and control servers, for example).

3. Even if they develop their own stuff, most governments have multiple arms which can use these tools (think about FBI, CIA, NSA, various military intelligence branches), and they tend not to share between them. This makes smaller government branches which don't have the resources and expertise of the others (think DEA, ATF...) buy from 3rd parties.

4. Zero days are a scarce resource, if I ran an agency I'd rather use someone else's every day and keep my own just for the special stuff.

In summary, it's exceedingly appealing for bodies like the Dutch police to use NSO tools and NSO's association with the Saudis and other provides a convenient masking to their operations.