#1) Software used to live on a disk you buy. That software doesn't change, until buy a new disk. Then it lived in apps, where you can update or not. Now often, it lives in the mythical cloud where changes can happen all the time.
Now, even on the disk front, things were always changing. Often for the better, sometimes for the worse. In the OS world Dos->Windows->Windows 95 were big changes, and the OS9->OSX change also huge! But now the changes are always.
#2) The entire software world is built on VC money. VC money is not looking for slow and sustainable growth. Or a happy userbase of 10k people. The VC world doesn't mind if 99 companies crash and burn trying to harvest the wind while building their sails if one takes off.
#3) Out of the VC/startup world, large companies must justify their existence and every team and every programmer on that team must justify theirs. No one ever stuck around by saying "everything is good, we literally don't need to do anything or acquire another customer, let's all cut out hours to 2 days a week, keep patching bugs and making security updates, be happy with our current level of subscriptions, and spend a bit of time in R&D to make sure we have other bait in the water too if for some reason our users stop liking this.