Producing and carrying offspring is far more costly and risky than just producing sperm. One excellent example of this is flatworm penis-fencing where they battle to impregnate their opponent while avoiding it themselves.

Some point in our evolutionary history as mammals some mutation made it impossible for one side to get pregnant at all and only able to impregnate others thus freeing up resources for males to focus on getting as many females pregnant as they could. This strategy also carries the danger of relying entirely on others to reproduce. If females develop the ability to select only female offspring and not males then this can eliminate y-chromosomes entirely, something that has been theorized to have happened more than once already in our evolutionary past until a y-chromosome able to overcome this selectivity happened.

There are entire books on the topic of male-female reproductive strategies and cost-benefit analysis at the genome level which I won't go into as I don't have a teaching degree.