This is actually not particularly true. In fact, the problem of unique pieces almost ended up sinking Lego because they were spending so much on molds and manufacturing of a single piece that was only used in one set. If the set didn't sell well, Lego could potentially lose money on the set overall because of all the tooling costs.
Back in the aughts, they redid their philosophy of having large single-purpose pieces and went to having those large pieces be replaced by subassemblies of much smaller pieces that were much more general purpose. That's when SNOT became huge in Lego's official sets.
As someone with multiple decades of experience with Lego, things now are much much better than they were back in the 90's and early aughts specifically because of this pivot that Lego did. There is something to be said about part count inflation, and how many of the parts nowadays are tiny little pieces rather than the big 2x2 or 2x4 bricks. And also, some sets and some themes do require their unique pieces. Friends has their little minifigures that are different from the standard minifigs. The Mario sets might need to have some specific pieces -- there's no standard 'mario mustache' tile. But overall, Lego has done a pretty incredible job of increasing the utility and decreasing the single-use aspect of many pieces.