It seems that more and more streetlights today are post-tops (the lantern sits directly on the top of the lamppost). Based on the comments of other members it's so worldwide.
Lanterns hanging on a steel cable between buildings over the centre of the street seem to have the perfect position but it's rather an ugly way and together with wall-mounted fixtures require unwanted drilling to the facade.
But why not outreach arms? They're a good compromise and they often were at places of the current post-tops and people (incl. me) seem to like them better.

A post-top is usually low and makes a bright patch at the footway/sidewalk underneath (or, it even illuminates its own lamppost) and lets the place between rather dark. Special reflectors (in case of HIDs) preferring sideward directions could help but they would also increase glaring. In case of LEDs a cobra-head seems to solve this problem by having sideward shining LEDs more dense.


So why so many post-tops? Are there only economical reasons - less steel (outweighed by luminous inefficiency) or something else? (Post-top lampposts can be made of a "foil" as there's no lever as with an outreach arm.)

In the USA, ornamental post-top fixtures are popular for making downtown areas look better.  We got nice black 100W MH post lanterns on our main street about 10 years ago to replace HPS cobra heads on wood poles.  I've seen street lights attached to buildings in photos of European villages, but that isn't generally done here.  Our streets are wider and there are legal issues and the need to get un-metered power to the lights or credit the building owner for the energy usage of the fixture.  Once in a while I see a fixture strung on a cable so it's over the middle of the street, but it's rare here.

Matt

A straight pole is simply just cheaper than a pole with a outreach. Here in Switzerland almost all new installations are using straight poles. Only on certain places brackets are installed, mainly for decorative purposes and on areas where a strait pole is not sufficent enough.

On narrow streets we still use laterns on span wires as well fixtures fitted on facades. Due to high maitenance costs overhead powerlines are getting rare so no more streetlights on wooden poles.

Deinitely it is an economic reason.
Lower and thin poles are preferred, because they more easily blend with the environment, so are "less ugly" than the high massive poles needed to support the truss arms.

The overhead wiring would be the cheapest (both to build, as well as to maintain - accessible for diagnostic and repairs, the most expensive part of any underground installation - the trench digging is not needed,...), but it does form quite ugly web most cities are trying to get rid of.

Yes, appearances drive putting wiring underground (street lights and suburban residential power) in the USA for the last several decades.  Our town had to run underground wires for the new post lanterns.  They used a directional drilling machine, which can make holes sideways in the ground to minimize digging stuff up.  Still must have cost a lot to put in, but looks nice now that it's done.

Matt