This is a common optical illusion with bike lanes. Bicycles are a lot smaller than cars and are 100x less likely to encounter traffic congestion, so bicycle traffic always looks much lighter than a similar number of cars.
The only way to be certain about traffic numbers is to do a traffic survey, where you actually go and count the traffic amount during rush hour.
Heres an example of a traffic counter where bicycle traffic looks a lot lighter than it actually is.
Also, even if a bike lane is underutilized, it is often a first step towards a proper bike network. Once a bike network reaches a critical mass, the traffic grows exponentially as people feel safe biking and new kinds of business is enabled by bicycle transport.
In countries like the Netherlands it is very rare to build a road WITHOUT a bike path, and I think that should be the standard. We should be asking why it didn't have a bike lane when the road was first built.