It might feel that way to you, but it seems you haven't bothered to look into the history of why certain classes are protected against discrimination in housing, employment, medical treatment etc.
One of those reasons is to fight the tendency for discrimination to create second-class citizens[1]. Along with our society's past and its contemporary history, we also have a several millennia of written history to look back upon to see just how easily and willing we are to make life very bad for people who are discriminated against.
And sure, the existence of those laws might be arbitrary. But the laws themselves are specific.
You say that they "were passed in response to actual and widespread discriminatory behavior". That's true. But they wouldn't have been passed, notwithstanding discrimination, without enough political support (of one sort or another).
I mean, there's also been discrimination in health insurance rates based on preexisting conditions. And gender-based discrimination in vehicle insurance rates. The Affordable Care Act more-or-less restricted the first. But the second is still the norm in the US.
There is plenty of ways to lawfully discriminate in this country. You can even still have discriminatory policies in employment if you can show it directly relates to the job.
Also, when it's about stuff like housing and services, there's not much basis for discrimination. Except for providing access to those with disabilities. And that seems fair.
When it's about employment, even if there are data that might justify discrimination, it's all about statistical distributions for populations. So there's too much uncertainty when you apply it to individuals. And there's also the fact that untangling innate/genetic and developmental/sociological factors is impossible.
For health and life insurance, basing rates on age and preexisting conditions clearly makes economic sense. Older people will likely cost more than younger people. And people diagnosed with cancer etc will likely cost more than people generlly. But for health insurance, there are social justice arguments that discrimination is unfair.
For vehicle insurance, it's undeniable that young men have more accidents than young women, and middle-aged people generally. And that old people people also have more accidents. At least two factors distinguish that from health insurance. First, there's the sense that people can choose to drive more carefully, and have fewer accidents. Also, there's the argument that driving isn't as essential as medical care.
Actual past experience with specific, widespread, and demonstrably harmful discriminatory practices.
Widespread discrimination in housing during the 20th century -- and the negative effects that had on certain communities -- resulted in laws prohibiting discrimination in housing ads.
Widespread discrimination in employment during the 20th century -- and the negative effects that had on certain communities -- resulted in laws prohibiting discrimination in employment ads.
BTW, these categories also make sense. Housing (i.e., schooling) and employment have a huge impact on your life outcomes in the USA. Choice of hair product, not so much.
For instance, travel services must be free from discrimination:
> Everyone has the right to freedom of movement and residence within the borders of each State.
Or buying property
> Everyone has the right to own property alone as well as in association with others.
Or the issues of this thread
> Everyone has the right to work, to free choice of employment, to just and favourable conditions of work and to protection against unemployment.
Or general social servies
> Everyone has the right to a standard of living adequate for the health and well-being of himself and of his family, including food, clothing, housing and medical care and necessary social services, ...
specifically education
> Everyone has the right to education.