The easiest way to prevent crime is to simply make it difficult to occur successfully. Most homeowners typically wouldn't leave their front or back doors open and unlocked, so keep that same mindset in regards to the garage. An open garage door is like an open invitation for criminals to waltz in and take whatever they want.
The most likely time for a homeowner to leave their garage door open is during the day when they're out doing yard work. For those within viewing distance of their garage, this is no problem. But, yard work typically takes homeowners into the backyard or around the side of the house for at least a few minutes. And it only takes a few seconds for criminals to run in, grab anything valuable and be gone before anyone realizes it.
The best way to prevent this sort of theft is to keep the garage door closed whenever no one is home and also when doing yard work. Get to know your neighbors and help keep an eye out for any suspicious vehicles or persons. No matter how safe you believe your neighborhood to be, it's best to be safe than sorry.