Building takeout archives is incredibly expensive for Google. Do that just a few times and you might have wiped out years of profit Google made off you. Thats because most data in your Google account (eg. a random email or a photo you took in 2013) is only accessed once and then never again. Storage systems are designed for that. Yet when you go and try and access every single piece of data ever, you're putting more load on their storage systems than years of regular use.
Thats part of why it takes so long to build the archive - they do it at low priority and slowly so as not to overload the service for other users.