Trains have to stop at rail stations though, so there is no choice other than to try and make the station as pleasant as possible. Buses have the flexibility to stop on the road.
Buses have to stop as well. The parallel is there - an average bus stop on the road is the equivalent of a rail "halt". In larger places, the station size and facilities will reflect the demand and footfall etc. To illustrate.... you wouldn't expect Weston super Mare to have a train station comprising a couple of platforms, some iffy shelters and a self service ticket machine. However, that sort of facility would be fine for Worle station (located on the edge of the town).
No, you'd expect Weston station to have toilets, waiting rooms, refreshments. However, Weston's bus passengers have to manage to navigate around a number of roadside stops in multiple locations. Not so bad for local services running a decent headway but less good for those running only half hourly, hourly or worse. I do agree that bus stations won't necessarily have ALL town centre passengers. However, we're talking about a focus point for the majority of passengers in the town centre.
Perhaps one of the significant differences on the continent is that as we've ripped up a lot of our railways and haven't invested in them (or in light rail), buses have to pick up much more of the inter-urban workload so bus passenger waiting/dwell is for longer.
Not quite sure what you mean by an enclosed bus station? I assume you're on about a single concourse rather than a fully undercover arrangement.
Fair comment on the investment in physical infrastructure like bus stations rather than other areas. I've mentioned the Dinnington and Ossett examples but could easily point to places like Wigan's new bus station that is both impressive and ridiculously large. PTEs are most culpable but even places like Newark seem a bit OTT - could be a thread in itself!
It is just my personal opinion but I think you have to be pragmatic about bus stations. Some places don't really need much more than a couple of bus shelters and some nearby public toilets - the minimalist option. However, some places would benefit from something that is central and appropriately sized and specced. Where possible, try to put them near the rail station - Hull is a great example of what to do (whilst Cardiff is the opposite).