This is not the ‘permanence’ justification that is often heard and easily disproved (i.e. where are they now if they were so permanent?). But it is related, once laid down, tracks are harder to move than buses, and tracks are more expensive, so it is harder to make routes circuitous. Many bus routes look like they were designed by drunk transit planners. One local bus the 370, which runs near my office and my home is so circuitous it is faster to walk even ignoring schedule delay. (It is not quite faster to walk end-to-end though, walking time is 2:30 vs. 1:14 on the bus, so the effective bus speed, assuming schedule compliance, is about 9.6 km/h vs. 4.8 km/h walking.) I have written about this before in Minneapolis, (and nearby Rosedale) and circuity is hardly an unknown problem.
Now there are undoubtedly reasons for every indirect deviation that diverts buses from the straight and narrow. However, every circuitous zig also loses passengers, and bus routes in the US are much more circuitous than travel by road. Serve this building, serve that one, cover this street, reduce pedestrian walking time.
In contrast, trams in practice are much more straight-laced, paragons of transit routing virtue. The historic Sydney Tram Map, as this map in wikipedia shows, gives a sense of routes that were pretty much as direct as possible.
Now it can be argued this particular bus provides and east-west service that no tram did, which is true in part. But that doesn’t mean trams could not. It also could be argued that almost no one rides the 370 end-to-end. Though I have not checked the Opal data, this is probably true as well. But a well-structured suburb-to-suburb transit network (my fantasy map is here, Jarrett Walker has done this as well) could avoid this. To be fair as well, the Sydney frequent network is not nearly as circuitous as the 370 bus, which has a roughly 20 minute headway