Because of a variety of reasons:
- Strict scrutiny is an invitation for frivolous suits. Because everyone (who has no clue) thinks "strict scrutiny means the right can't be infringed at all". That just means a ton of work for federal courts to get rid of silly cases.
- Because strict scrutiny isn't actually strict. As much as I dislike Justice Thomas (or more accurately, his inane reasoning), one of his observations (in an opinion or dissent) is that strict scrutiny is applied highly inconsistently, and he is right on that.
- Strict scrutiny doesn't mean that the right is sacrosanct. It only means that the state has to do their homework when restricting it. For example, there is a statistic out there that roughly 2/3 of all laws that are challenged under strict scrutiny over cases of religious freedom survive in court.
EDITed to add: I would love it if the US legal system had basic human rights actually written down coherently, had some serious respect for those rights, and employed a real form of "strict scrutiny" in evaluating the power of the state. But that's not the world we live in. You go to war with the legal system you have, not the one you want. And in our current one, asking for "strict scrutiny" gives you a pacifier, not something useful.