Because CONS cells aren't used only for lists, but are fundamental data type used as building blocks for few other things.
As such, they are required to hold any value or reference possible, and aren't exactly amenable to different format.
There is one technique which had somewhat large use, CDR-coding, based in the idea that you can tag a cell in a way that tells the system that the next field isn't a reference to a CONS cell, but the CAR of one, and thus linearize lists in memory. However, doing it fast requires a custom cpu so you can resolve this fast in transparent way, as well as it turned out to not give much benefit. However, widening gap between cpu and memory might make it attractive again...
As such, they are required to hold any value or reference possible, and aren't exactly amenable to different format.
There is one technique which had somewhat large use, CDR-coding, based in the idea that you can tag a cell in a way that tells the system that the next field isn't a reference to a CONS cell, but the CAR of one, and thus linearize lists in memory. However, doing it fast requires a custom cpu so you can resolve this fast in transparent way, as well as it turned out to not give much benefit. However, widening gap between cpu and memory might make it attractive again...