Which might make sense. Perhaps more broadly, they see the advertising entities as a whole that is complete, e.g. the value of these is strong, and not likely to be anything more than incremental (in the case of Youtube, that increment is still likely thousands of percents) nor to need any capital or debt for the foreseeable future.
Fiber, cars et al could be spun off, which would create share holder value without dividends, and allow these units to leverage debt rather than capital. Cars, Fiber, the WiFi balloons, could all be spun off at a point where they need capital and debt, and not drag down Alphabet.
Fiber with an IPO and access to debt as its own needs dictate might grow a lot faster than fiber waiting for capital from Google.
Maybe that is Google's plan: get them ready for IPO, then set them free and take an interest?