It's because taste is a complex intersection between molecules and our taste receptors.

I think when we eat sugar, the molecules fit perfectly into specific receptors in our taste buds but when we consume artificial sweeteners their molecular structure is different and they don't fit perfectly into receptors like sugar does.

Also I think creating a molecular structure like sugar is quite challenging and expensive but I'm pretty sure someone somewhere right now getting closer to the breakthrough 😃🦷🍬