Zen 2 compute chiplets are 8 cores, the 12/16-core desktop parts have two of them. Right now the Ryzen 3000 APU's are still using the classic Zen(+) cores with Vega graphics on the same chiplet, hence the limit. The next-gen APU's will likely feature the Zen 2 core chiplet plus a Navi GPU and give you 8 cores on mobile.
How would you feel about a 10-core Comet Lake?
I hope AMD starts using more than 1 chiplet for laptops. Until they do an AMD laptop is limited to 4 cores.