Because they're a little more conservative and build their kit with longevity and durability in mind. Most would agree that SRAM kit, as great as it is, isn't as durable.
I don't agree. I have extensive use of top groups from both Shimano and Sram (will be adding Campy soon) and I have settled on Red 22 mechanical as my favorite group for weight but also prefer its simplicity of mechanism and lack of maintenance. I still have never broken a cable on any Sram shifter. I have broken several on Shimano dating back to 9 speed. I also have first generation Sram Red groupsets on two bikes that have been through the wars. One is on my winter bike now and the other on my travel bike. Both groupsets perform as new. Out of 4 Sram groupsets, I have had only a single part failure - a broken RD spring on a first generation Red RD. The RD was instantly replaced by Sram despite being years out of warranty. I would be considered a heavy user. I ride all year round and average 5 days per week throughout the year. My typical ride ranges from 50 km to 120 km with some longer in the summer. I live surrounded by rolling terrain with some moderate hills that is a torture test for shifters - constant gear changes are required.
I am sure there are all sort of Sram failures, but I doubt the failure rate would be statistically different than any other brand. While my usage is only a single data point, over the years, I have had many more breakages and malfunctions (usually shifters) on my Shimano equipment despite it seeing far less usage.
Sram Red is lighter because it is better engineered, or at least engineered to favour low weight, not because it is compromised in some way. There certainly is no durability issue that should factor into a purchase decision. For most people the stuff will be obsolete before it wears out.
When we ride disc brakes the whole deal of braking is just like a leaving a fart. It happens and then it's over. Nothing planned and nothing to get nervous for.