Mostly because the market has not demanded them, or at least the marketing departments at the camera and lens makers have not perceived much of a demand for them.
Most photographers who choose to use cameras with sensors smaller than full frame do so due to cost considerations. This includes not only the cost of camera bodies but also the cost of lenses to use on them. In 2021 the cost difference between full frame and APS-C sensors is nowhere near as wide a gap as it was just a few years ago.
But the camera body is only half the equation.
A lens like the Canon EF-S 55-250mm f/4-5.6 STM can be made much more cheaply and yet perform just as well within the smaller image circle area needed by an APS-C camera as a lens like the EF 70-300mm f/4-5.6 IS. Due to the smaller image circle needed, the lens can also be lighter.
For those who choose smaller formats for reasons other than cost, the main one is compactness and portability. Wide aperture, constant aperture zoom lenses are typically not the lightest and most compact lenses available within the range of possibilities in any format. Putting a relatively large and heavy lens like the Sigma 18-35mm f/1.8 DC or Sigma 50-100mm f/1.8 DC, compared to other lenses available for the APS-C format, on a smaller, lighter body kind of negates the advantage of a smaller, lighter body.
While it may be true that a 50-100/1.8 on an APS-C body is smaller and lighter than a 70-200/2.8 on a FF body, there doesn't seem to be enough demand for such lenses to motivate manufacturers to offer them. Buyers willing to pay what such lenses cost are usually going to buy a FF camera at some point anyway, particularly since most newbies seem to think the camera is more important than the lens.
Remember, there's no such thing as "true equivalence" when comparing different formats. It's always an approximation. The only way one could have even theoretical true equivalence is if we could scale the wavelengths of various colors of light (and the energy and mass of photons) by the same factor as we scale everything else.