Again, note that legacy metros are largely confined to older cities, which have them for historic reasons, have never had the option of having (regional) mainline railways serving the town centre, which instead only served the edge of town, metro was seen as a way to get people between the town centre and the outlying railway station.
Here is a map of London's underground, and note that the tube is largely north of the Thames. And most of it dates from before the first world war.
The north part of the largest city in the country that gave the world heavy rail was ironically already heavily developed when the railways came, and trains simply could not run at grade (be it in cuttings or on embankments) within north London. The solution was to run underground and there was a short tunnel, with steam trains, which allowed trains to run through the town centre. But the trains needed to be fitted with condensors and I imagine this tunnel would have needed much more ventilation than a road tunnel. But more tunnels were needed to serve more of the old part of London by trains, often needing deeper tunnels. Steam trains turned out to be inadequate for extensive underground running, so it had to be electric. As surface rail was still unelectrified, with surface trains still steam hauled, the new deep tube lines had to be physically separate from surface rail.
Other old heavily developed cities, on mainland Europe soon followed suit.
But all Australian state captials, being developed later, could have mainline railways serving the town centres (now CBDs) and therefore had no need for rail tunnels below city streets in the days of steam. Furthermore, both in Sydney and especially Melbourne, surface railways used by suburban services were electrifed before underground railways were needed, making it practical to simply extend the existing network through tunnels in the CBDs, and in the eastern Sydney suburbs.
Heavily developed cities like London, Paris and Berlin all had established metro networks by the time that suburban services began on mainline railways, let alone by the time these were electrified, and these metro lines, with their classic metro legacies, simply could not be intergrated into the suburban railway networks. In addition to small bore tunnels (probably predating tunnel boring machines) and close station spacing, the Paris metro, for example, has tighter curves than found on mainline railways.
All 13 of the classic metro lines in Paris were originaly interoperable, later some of them were converted to rubber tyred metro, partly because of the tight curves, while orgininally, the whole network was planned to be converted, that plan was abandoned. That's the real reason why Paris has some lines incompadible with others, even line 14 trains could run on other lines under manual control.