Why does Canon not make such light zoom lenses anymore ?

About 20 years ago I bought 2 excellent EF zoom lens - a 28 - 80mm f3.5-5.6, and a 70 - 210mm f3.5-4.5. Both were of the highest optical quality, though they were not top of the range at the time. Both produced many pictures that were subsequently published. Neither lens was particularly heavy, certainly not of the weight of present day lens. I now have an EF 70 - 200mm f4 L series lens (not IS), but in truth it is not as good as my old 70 -210mm which I still sometimes use. It is lighter, shorter, and, I find, better balanced than my modern L series lens. Unfortunately, after nearly 20 years of relatively heavy use, there is some slippage in the zoom mechanism which I am told is beyond repair as the parts no longer exist, otherwise it would be in full time use.

The only reason I use my EF 24 - 105mm IS f4 L series in preference to the 28 - 80mm is the greater range that it provides, the IQ is really no better and it is very considerably heavier.

I remain unconvinced that modern lens are necessarily always providing better IQ than those previously available in terms of optical quality, especially for FF. The increase in weight may be a corollary of the development of IS, which has certainly improved the possibility of getting the shot you want,  and the use of better methods of mechanical operation which have led to greater speed and accuracy in AF. There is no doubt that these two improvements are frequently worth the increased weight of the lens and result in us getting better images more often.