domain name system - DNS hosting design - should secondary server not be spread over countries, networks etc?

My DNS registrar and DNS provider recently had a long outage, rendering all my domains unusable (email, own+client websites etc).

They have 3 DNS server, who are all in the same co-hosting facility!

I know just enough about networking to make my spidey-sense supertingle, but not enough to condemn this. Is that not an atrocious design?

Should they not have been spread across lines, networks - even continents?

(Source: https://help.hover.com/hc/en-us/community/posts/115007805527-After-recent-outage-what-are-you-going-to-do-to-fix-your-network-design-problems-)