2.4 can stream 3-4 x4k video at the same time and 15+ 1080p ..
On paper, maybe, but most of the 2.4 Ghz wifi implementations aren't great. 802.11b is only 11 Mbit max. 802.11g sounds like it would do the job since the spec nominally supports 54 Mbit, but in reality its limited to 31 Mbits effective throughput. 802.11n is the only really viable choice at 2,4Ghz, but you're still capped around well under 70 Mbits with a single radio channel.
Really, the 5 Ghz requirement is there not just for simple bandwidth but because of less interference in the 5 Ghz band, both from other wifi base stations and Bluetooth. I'm sure it could have been made to work at 2.4 Ghz, but it would have been poor from a reliability perspective.
I also think its a means of getting more CPU horsepower in the head unit by association, if you will. Any SoC implementation used in the head unit that supports 5 Ghz wifi is also likely to have reasonably good computing power, which also increases reliability/responsiveness. So many car head units have absolute garbage computing power in them, and much of this was spec developed years ago, too, when it was even worse than now.
The TL;DR is that CarPlay probably could have "worked" at 2.4 Ghz, but it would have been glitchy and slow and vendors would have shoehorned it onto sub-par head unit technology, resulting in a lot of consumer griping. Apple made 5 Ghz a standard for basic reliability for reasons beyond basic bandwidth math.