Your standard actor doesn’t get to negotiate a contract. That they sign the union contract makes their compensation fairly reasonable.
In any case, by your own words, it is fine for the studios and the union who speak for the actors to negotiate a closed shop.
Residuals are profit sharing. Commercials, which use more actors than series, that run all the time make more money for the agency. Why not give some of that to the actors? Should CEOs get paid the same if their companies do $1 million in sales versus $1 billion? Widget makes would be like extras, who don’t get residuals. The widget designer might will have a bonus plan which gives more money for successful products.
Not to mention that residuals keep production costs down, paying your average actor a relatively smaller amount in return for a possible bigger payout later. Increase salaries to a living wage level and there will be fewer productions and thus less work for everyone, including crew who don’t get residuals.