I'm glad things stay put for you--because sometimes round my house I can't find anything when I want to lay my hands on it! But seriously...
The easy answer to your question is inertia. (Look it up in a reference book or talk about it with someone who enjoyed high school physics.) But 'inertia' isn't really an answer at all--it's really just a description of 'why things stay put'. No one actually knows the answer but there are some theories. Here's my favourite.
Some American scientists recently extended the ideas of a famous Russian scientist who is now dead (Andrei Sakharov). I don't understand it fully (the maths is beyond me) but I'll try to explain as best I can. Try this for brain-boggling!
A 'thought experiment':
- Imagine an object--for example the ceramic model of Alice (in Wonderland) on my desk. It's not moving (relative to the desk, the room, the Earth, etc).
- Imagine we sit Alice in a very strong glass case that is 100% slippery.
- We carefully suck all the air out of the case. A vacuum (nothing at all except Alice) is left.
- Well, most physicists think that a vacuum is really a 'zero-point field'. Tiny bits of matter (smaller than atoms) pop into existence and disappear again incredibly fast. (By the way, that's part of an explanation for how black holes might be able to emit radiation...but that's another story.)
- Now put your hand in the special glove that fits in one wall of the thick glass vacuum chamber. Give Alice a tiny push. If she was made from lead she would slide slowly away. Because she's made from clay she slides away quickly. The more matter she is made from the more there is to drag through the sea of quick-n-tiny 'virtual' particles in the zero-point field. (Imagine you throw a rock through air then go and get it and throw it just as hard underwater. It's got more to push against underwater so it doesn't go as fast. Well, it's a bit like that, only in reverse.)
- If there is drag, then perhaps that's what inertia is. Or something like it.
Confused? So am I.
It's a deep question. Alice wants to know the answer, too.
By the way. Not everything stays put! Liquid helium when it's cooled to near absolute zero doesn't stay put. All by itself it climbs up out of any container it's in and slides down the sides! Plenty of mass. Little inertia!