It was in development for a long time at a critical point in history. It seems to have started off as a software-first project (or possibly even software-only), then switched to hardware-first near the end of development. And not every effect was implemented in the hardware renderers.
Half-Life has a lot of grid textures that really need bilinear filtering and mipmapping to look decent, and they look even better with anisotropic filtering.
Many other games that released a little later (like Decent III) actually fully removed their software renderers before release because games needed to be hardware-only if they actually wanted to compete on visual quality.
It was in development for a long time at a critical point in history. It seems to have started off as a software-first project (or possibly even software-only), then switched to hardware-first near the end of development. And not every effect was implemented in the hardware renderers.
Half-Life has a lot of grid textures that really need bilinear filtering and mipmapping to look decent, and they look even better with anisotropic filtering.
Many other games that released a little later (like Decent III) actually fully removed their software renderers before release because games needed to be hardware-only if they actually wanted to compete on visual quality.