Why don't smaller powered airplanes have better lift-to-drag ratios?
There are plenty of smaller power planes that achieve those numbers; motorgliders.  And motorgliders with L/Ds in the high teens and low 20s are pretty efficient cruisers.

So the real question is; why aren't all light aircraft made to be like motorgliders?

Well, motorgliders have their disadvantages.  The long wing span is a problem fitting in on the ground.  The long wings are heavy, which limits payload.  The long wings result in slow roll rates and general controlability problems.  So if you are going to live with the hassle of long wings, it has to be really worth the trouble.  

And probably the biggest one; the long wings stop being an aerodynamic advantage and become an impediment when you want to cruise at significant margins above speed for best L/D.  This is the case for aircraft that cruise at low altitudes if you want to cruise at 2-3 times max L/D speed efficiently.  In this case, &lt; span = better.  WW2 fighters modified to be optimized for low altitude combat had their wing spans reduced.

Airliners are more like the motorglider, and benefit from the L/D benefits of high aspect ratio because of their operating environment.  They cruise at very high altitudes where indicated airspeeds are low because of the thin air.  At FL400, an airliner might be cruising in the thin air at only 280 kts indicated, maybe only 2X its flaps up stall speed (going 500+ true, but it's the indicated airspeed that matters - if you could stick your hand out the window, you'd feel a 280 kt wind blast, not a 500 kt wind blast). 

It's cruising a lot closer to its max L/D speed than the low altitude airplane that flies at 3-4 times its stall speed, and so it benefits from the span and area like a motorglider.  To take that to it's most extreme, you have the Lockheed U2, which needs all that span and area to make it up to 75000 ft (where the indicated airspeed is only about 80 kt going, say, 200 kts true or whatever it is).

You do see some aircraft like the Diamond light twins with quite long aspect ratios for what they are.  This is done mostly for rate of climb. because in a light twin, single engine climb performance is critical, and the high aspect ratio really helps (climbing close to max L/D speed) and that's factored into the balancing act of deciding just how long the wings need to be for good single engine performance without giving up too much controlability or taking on too much weight (helped there by composites) or too much space on the ramp.