That may explain why roses smell nice, but many wildflowers smell nice too.
> By and large, roses smell nice because human bred them to.
That theory seems unlikely to me since wild roses smell good.
I would say that the modern roses that don't smell good have instead lost their smell because they were selected on other criteria.
If one such characteristic is pleasant smell (and why would it not be), then the answer is obvious. By and large, roses smell nice because human bred them to.