Mobile phone questions

First off are signal dropouts caused by moving from one
cell and then into another? and when you move from
an open area to say inside a building or behind a hill or
a 'dark spot' where there is less signal penetration....

Would allowing you to roam between networks when the
signal level drops below a certain point solve the problem
and if it would why aren't the networks allowing this to
happen? It surely wouldn't be a cost to them to allow for
this would it?