I get lots of 20lb propane tanks and grills, most are not Weber.
Here’s what I see…
First if you follow the TVWBB, have more than 1 grill and grill once per week or more you are the 1 percent of grill owners.
Most grill brands don’t have any propane tank measuring devices as a rule, only Weber. The exception is a Weber Q3xx
Most grills built in last 20 years hide the tank behind a panel or in a cabinet probably for aesthetics. If the tank is hidden and below your waist you are not going to see readings on a scale or tank gauge easily. Bending down seems to invoke a cost/benefits analysis as we age.
The tank mounting scheme on all grills is not ergonomically friendly. The tank needs to be upright so gas is discharged not liquid. The tank needs to be below the manifold. The tank should be secured for safety reasons. These 3 things leave you with an awkward “ball” of about 37lbs that’s below your core body strength to deal with. So most grills use a tank well or a bracket where you can kind of drop the tank in. Not ideal but easier than going “in and up” to hang from a hook or scale inside a cabinet.
Most of the tanks I get are expired, pre 2012. I see pressure gauges were popular on tanks before about 2004ish, around the time grills transitioned to cabinets and panels and away from tube steel framed open carts.
My conclusion is most people are just guesstimating what is in the tank and hedging with a spare tank which is very easy now that tank exchange is everywhere. You can’t see the tank on most grills so a tank scale or tank gauge is of little value to most people. If they were a must have the grill makers and aftermarket grill parts makers would respond. Most grills, the 99 percent, are used very little so 1 tank worth of propane is enough for a year in the 4 seasons part of the country.