This doesn't make sense to me. Why aren't they concentrating on their Z line and getting more native lenses out for those mirrorless cameras?
It is not as simple as you seem to think it is.
Because no company can ignore their customer base.
The entire customer base will not instantly switch from dSLRs to mirrorless. Some will, many will not.
If the customer base is slow to migrate to mirrorless, Nikon and Canon have to support them, until they get enough to switch.
If the camera I need is not yet available in mirrorless, I am still going to use my dSLR.
Both Nikon and Canon's 1st generation mirrorless had holes in the system; no 2nd card slot, no portrait battery grip, no high end pro/sports camera, etc.
If I bought a dSLR system in the last 5 years, I likely am not going to chunk away several thousand dollars of gear, to switch to mirrorless. I am going to use my dSLR as long as I can, and I expect to have lenses for it.
The dSLR camera owner is a captive market, for lenses.
If they completely switch to mirrorless, and drop support for the dSLRs, that would badly upset the customer base (pi$$ them off).
The current dSLR user is more likely to stay with the same brand when they switch to mirrorless . . . unless you badly upset them. If you do badly upset the customers, they may jump to a different brand, which is the last thing a manufacturer wants.
I know one formerly LOYAL Canon user who becamed so POed at Cannon, that when she decided to upgrade from APS-C to FF, she did not even consider Canon (dSLR or mirrorless), and went with Sony.
This is the problem that both Nikon and Canon have. In this transition period of dSLR to mirrorless, they have to BOTH, take care of their existing dSLR customer base and expand their new mirrorless system, at the same time.
Should Nikon and Canon have moved to mirrorless sooner, and not let Sony get so far ahead?
Yes, but hindsight is 20/20.