Polo Bikes

It's not about that, there is no point in having a horizontal top tube, it increases weight and decreases stiffness of the frame. As far as I can see, the only reason horizontal top tubes ever existed was because of industry standard lugs back in the day, and following from that, traditional aesthetics.

I'm not a frame builder though, someone please verify/dismiss my ramblings.

Nothing to do with lug availability - fillet brazing is not exactly a contemporary innovation. People used to build frames by hand and offer a wide range of sizes (say 3 or 4 top tube lengths for each seatpost length). It's much less time consuming to do this with a horizontal top tube, especially when top tube length is often considered the starting point of frame design. Plus, 99% of people who ride bikes do not want/need the lightest/stiffest frame they can get their hands on (and this is without getting into a conversation about whether or not the weight/stiffness difference actually makes a tangible difference).