I think they wanted to keep the monster stat blocks to a minimum to make the game run faster, rather than go back to 3e style stat blocks that had everything.
The downside is monsters suck at everything except fighting, and they're even expected to lose at that.
At low level, expertise is good but monsters typically still have a chance even with a flat d20 roll. It's not until proficiency bonuses go up to +4 and beyond that it becomes truly hopeless (specifically in combination with Reliable Talent), and at that point the low-level monsters aren't supposed to be a challenge anyway.
Basically, if you have a high level Reliable Talent Rogue with Expertise, they need skill challenges that not even other PCs are going to have a chance at succeeding at, while still allowing those other PCs to defeat the challenge in some other way.
For example, maybe the McGuffin is sitting on a pedestal in a brightly lit room, no cover, bubble wrap covered in flour on the floor, and guarded by some Big Bads. These factors make it impossible for pretty much anyone to sneak in there, but your high level rogue might just be able to pull it off if he gets help from his friends (distraction, spells like silence, invisibility and fly, etc.).
That all said, I do think it's unfortunate that high-level skill challenges are more difficult to design due to the massive gap between average PCs and reliable talent experts. It will either be impossible for everyone but the rogue, or the rogue will basically auto-succeed.