Because apps are running in the background and you have not run an optimization. Go to settings/device care and tap the optimize now option to clear your RAM. The select auto optimization and set it to restart on at least a weekly schedule.
Really need a lot more information. You can't think of RAM free space like you do storage free space. RAM makes your apps faster in both opening and fluid operation.
As long as you have about 20% free RAM you are ok. A tool like Samsung's Memory Guardian (part of Samsung’s Good Guardians) can show you this information in the History tab.
