Because if you have a need to regulate traffic through your node there is one, simple, perfectly effective way-- blocksonly. Any other way is ineffective (dramatically so if you wish to reduce traffic, as filtering generally doesn't) and has collateral damage potential.
From the discussion in public the motivation to do otherwise is an attempt to regulate the conduct of third parties. This is fundamentally authoritarian, it would still be authoritarian even if implemented in a distributed way. E.g. if a theocratic populist movement acted to prohibit sex for any purpose except reproduction (as advocated by the most prominent filter propents) such as by public stonings of people caught fornicating it would be just as authoritarian as if established by a dictator. In my view the fundamental nature of authoritarianism is people who believe they know better to such an extent that they actively interfere with the consensual conduct of third parties. Historically most authoritarianism has taken centralized forms, but this is partially just an implementation detail similar to how cultures have adopted representative democracy over direct democracy. Centralized authoritarianism is itself normally via a group like a state government, but just one with restricted membership. Technology can enable distributed authoritarianism like the cancel culture of filter proponents.
More importantly, I disagree that there is any meaningful democratization here-- to have any significant effects on the behavior of third parties, some external mechanism must coordinate the content of filters. Were this not the case, you could simply say "my filtering node software exists and is available, problem solved!" -- but you're not doing that, because to have any effect (to the limited extent that you can) you essentially need to convince everyone or at least most people to apply the same restrictions.
The fact that a mechanism isn't proposed here just obscures the matter because one will arise out of necessity (or, alternatively, the proposal would just not be used to a meaningful degree). In essence the proposal (or ones like it like the one being developed in knots) are technological instruments of authoritarian censorship. Sure they don't have all the components yet to complete their natural conclusion.
> which is that the core maintainers decide all the defaults
Defaults? well duh, yes any author of software decides its defaults and that is unchanged in this proposal. Nor does it change persons own ability to change their node behavior, as adjustments to policy are quite simple and with the LLMs that power most filter advocates arguments even a non-programmer can adjust them. What it does accomplish over that is the ability to take a live feed of censorship rules from a third party.
Why doesn't core ship blocks only? Core attempts to model what will get mined. My blocks only recommendation was for parties that prioritize conserving resources or avoiding various unconfirmed traffic over accurately modeling what will get mined.