In most U.S. ports, it's not Customs that makes the decision to inspect, it's actually ICE.
In the olden days, back when I worked as a contractor for Customs, entry into the U.S. went like this:
1. You went to Immigration Control first. Pre-ICE (US Immigration Control) checked your passport and entry form, OK'd you to enter, and then you reclaimed your baggage (whether you were traveling onwards or not).
2. You then had to clear Customs, which looked at your itinerary (e.g where you'd been), your bags (i.e were they bulging, smelly, etc.) and your face and non-verbal cues to determine if you warranted a further inspection. If you did have something questionable, there were actually expert (!!!) customs agents available to determine compliance.
Today, it goes like this:
1. You go to Immigration (ICE) first. Based on your facial expressions, non-verbal cues, and passport history, they determine if (1) you're OK to re-enter the US, and (2) if you need further "assistance" with customs. They make a mark on your entry form, which you later turn in to a customs agent, to indicate if you should be further "assisted". The decision regarding inspection and possible seizure of goods is left almost entirely with ICE, who specialize not in Customs enforcement but Border Control.
2. You move on to Customs Enforcement, which looks at your ICE-noted entry form and either inspects, detains, seizes, or lets you go based on ICE notes on your entry form. If ICE didn't mark your entry form for further scrutiny, you move through Customs very quickly.
The reason for the change? Efficiency. Most people re-entering the US don't need any re-entry assistance, and Customs agents are otherwise very busy. Giving ICE the job of determining 90% of Customs work saves time for travelers and money for the government. But the downside is that most ICE agents aren't trained to sniff out the difference between a guy with handcrafted musical instruments made of foreign raw materials from a guy bringing foreign raw materials into the US with the intent of defeating embargoes and/or tariffs.
The point is, it's not Customs that are dim, it's ICE... and as long as it saves most travelers some time at the desk, it probably won't change.