Because Howie ideally plays 2/3 of the time at 1B. Giving Zim 5 or 6 mill to play the rest of the time is a lot. That's likely what happens, but it's more a reward to him as a Cornerstone.
If Rendon comes back, you have Kieboom and 2nd, Zimm and 1st, and Howie as the third option. Kieboom could also play short or third. So maybe you sign one more IF option, Dozier perhaps, or go with Difo.
Without Rendon, Kieboom starts at third, Howie at 2nd, Zimm and 1st, and you need two more bench players.
As much as I love Howie and Zimm, an infield of Kieboom, Turner, Howie, and Zimm isn't all that awe-inspiring.