Suppose you build a house. When it’s finished it’s shiny, new, and clean.
Then you live in it for decades. Stacks of old newspapers litter the floors. Sometimes a bunch of these get thrown out. You only do superficial cleaning, and crud accumulates in all nooks and crannies.
You also want to embrace the new. You enthusiastically follow each fashion trend in furniture, appliances, and so forth. A new chair arrives, but the old one is still in use because grandma gets back pains when she uses the new one. You buy new nice plates and cups, but the old ones are not thrown away, just in case. The new robot vacuum cleaner cleans the living room, but the old one is still in a closet for the cleaning the robot cannot do. Even your replacements of worn out stuff leave leftovers all over the place. Still more crud accumulates in all nooks and crannies.