Because Android multitasking is inefficient with memory use.
My Palm Pilot had 512 kB of RAM and it could multi-task properly. When I re-open an app on Palm, I could always get back to EXACTLY where it was left off. Palm OS saves the app state before swapping it out of the RAM.
Android apps have to completely restart from fresh after being closed. It multi-tasks by keeping apps running in memory. But you are never going to have enough RAM for the apps if you use a variety of apps.
Samsung’s RAM Plus does something similar, but the apps are not designed to be efficient for virtual memory.