It’s because Remastered uses bilinear filtering to scale the image when using the original assets as opposed to your monitor just doing its own either analog (on a CRT) or subtler interpolation-based scaling either from the LCD monitor itself or the GPU.
Bilinear filtering looks far blurrier, and without any kind of filtering you have an uneven scale that looks jagged and distorted. Changing the scaling filtering to a simpler interpolation would yield much better results, but they’ll never do it.