Importing a list implies there is something to consume the list like an adblocker.
While this could and I think should be built into the browser having a complex enough addon system to build something like ublock origin ensures that you can always use best in class tools. As an example adblock plus was the best option years back until ublock origin came into being.
ublock origin uses less resources, improves latency, and blocks some things that adblock does not.
Had your hypothetical browser for techies standardized on building adblock plus into the browser you would be stuck with it for some years until they moved on instead of clicking a link in an addon store.
Complex tools always end up being about the ecosystem because core developers neither have man centuries to scratch every itch nor a monopoly on good ideas.
Insofar as you have an ecosystem distributing it via a central store or stores is a proven idea. You can make an informed decision to trust a store which can vet individual developers/companies and also make the decision to remove people who later abuse that trust.
Example stylish was used by many users then sold to scummy people.
Having your users browse the entire web and make the decision on the spot as to what to trust to run on their computer has failed so many times its hard to imagine it needs another try.
It's all well and good to imagine that a sandbox should be used to limit the access an individual app or addon is allowed to have but such cages always have weak hinges somewhere and furthermore people often want at least some apps to have relatively expansive permissions so that interesting tools can be built.
This leaves the users vulnerable to your hypothetical bad addon requesting permission to own the world and the user reflexively clicking yes.