1) I have experience designing both; and I believe the reason is most engineers start out learning only one at their initial job(s) out of university. From there they grow into designing what they know. Later in their career they may be given the opportunity to work on a large bridge or building but the barriers to self-teaching the design details outweigh the income of doing so.
That said, I imagine many bridge engineers actually do a fair amount of building work simply based on the volume of bridge work available versus buildings.
There's also something to be said for the ethical concerns of practicing outside your expertise. Kind of making this a Catch-22.
2) Almost exactly what Lom said; buildings are unique structures with varying budgets, goals, clients, challenges, etc. Bridges are spanning something for vehicles and pedestrians every time.
Thus, I tend to dislike the repetitive nature of bridge work. I also dislike that bridge design is harder to self-teach than buildings. A lot of knowledge is out there but to find it you kind of have to know what you're looking for and have someone else who has already done bridge design there to guide you.
Of course I don't hate bridge work; it's profitable and a decent challenge. I just prefer buildings because I feel I can massage a building design to get what I want out of it; with bridges I feel like I'm just following examples and standard details without any freedom to go outside the bounds of what came before.
It's worth mentioning that I'm a one-man shop with a few EITs; so my bridge work is limited to simple, single-span bridges, box culverts, and abutments. I've definitely done more building work than bridge work.
Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries
