As mentioned you shouldn’t “cross-chain,” which is using the big ring up front with the largest gear in the back (or using the smallest [granny] ring up front with the smallest gear in the back. Besides causing the chain to fall off in the front, it also causes both stress to the chain and wear to the sprocket teeth. Stress to the chain could cause it to snap unexpectedly. Wear to the sprocket teeth will eventually cause the chain to slip, especially when climbing or riding hard.
The big challenge when adding more sprockets in the back is that you’re trying to fit more gears into a finite space; nine-speed gears and chains are slimmer than eight-speed. If either Shimano or Campignolo designed a fifteen-speed cassette and enough riders saw an advantage, frame makers would eventually redesign their frames to accommodate the new gear. As you might guess, new gear is generally designed for pro riders and trickles down to recreational riders.