Most of today's R&B is being made like Hip-Hop. They're Hip-Hop beats, not really R&B. Hip-Hop beats are usually either 2-bar or 4-bar loops, in terms of the harmony/chords. That means there's not multiple sections in songs. So it's impossible to have key changes, or substitute chords or switching anything up. That's why it all sounds so static.
They do this for a few reasons: two-bar beats can be thrown together faster, you don't need as much experience or training to make those, they're much cheaper to make, and the songs tend to be shorter, making them more approriate for Tik Tok.
I realized how simple almost all of today's music is from learning older music on guitar and comparing that with new music. The older stuff often has alot of variation. The new stuff doesn't.