Because Half-Life has a specific "pixel art" style that gets blurred to mush in hardware mode.
Not to mention the liquid surface effects look awful and the lightmaps are not as pretty (I still think it's pretty fantastic that they even had colored lighting running in software).
Also, any texture that isn't power-of-two (and there are quite a few in the game) will get rounded down to look even blurrier than it should, though they did fix this problem in HL:Source. Quake 2 had a cvar to fix this, but if it exists in HL, I couldn't find it.
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I'll brave the storm to come, for it surely looks like rain...