>I thought it was because the stores can't press charges if it's a small thing, so the only way they can bring any action is to build a case.

Firstly, stores don't "press charges." A store may report a crime, but it is the state that "presses charges" and prosecutes alleged criminal activity, not the store.

Secondly, in the US, we have statutes of limitation[0] which limit the time in which criminal charges can be brought. These vary by state and by offense, but IIUC Petit (sometimes referred to as 'Petty') Theft usually has a one or two year statute of limitations.

I bring that up as, again IIUC, other countries (notably the UK) do not have such limitations.

IANAL, but I'm not sure if multiple petit thefts (usually misdemeanors) can be aggregated into a single charge of grand theft[0] (a felony). I'd expect that also varies by state. YMMV.

[0] Once again, this varies by state, but petit theft (larceny) is typically charged for stuff valued at less than $5,000.00, while grand larceny is for stuff valued at USD5,000.00 or more.