Labour law and unionism vary greatly from jurisdiction to jurisdiction so a blanket claim that "labor unions don't exist for lawyers" cannot be so cavalierly thrown out.
I also think "... working hours far beyond what's usually expected in other professions" needs some elaboration. A 60+ hour per week workload is not atypical for lawyers in Australia. Professional engineers would have similar workloads and intern doctors will often be given 48 hour shifts, albeit they will be allowed to sleep during quiet times (if any). Hmm ... maybe these guys do need a union?
Speaking specifically for Australia, there is no specific union for lawyers, however, depending on their industry, role and employer lawyers are eligible to join a large number of unions who will represent their interests.
As to why this is the case in Australia, that has to do with the historical development of unionism in this country. The first evidence of collective bargaining occurred almost immediately after British settlement in 1788 with convicts negotiating progressively lower daily furrowing quotas from the military governor. The fact that the colony almost failed due to food shortages may or may not be related.
The true birth of unionism was in the late 19th and early 20th century and was primarily based on manual labour trades and industries; not professionals. The creation of the Australian Labor Party, which remains one of the two major parties in Australian democracy, as a union controlled party alienated upper- and middle-class professionals who tended to identify with the conservative side rather than the progressive side of politics - it is generally not wise to kick away the ladder that got you to the top; particularly when you are still standing on it.
Since that time unions in Australia have generally been trade/industry based or public sector unions - they are also less of them than there were as it has been bi-partisan policy to reduce the overall numbers by amalgamation. A full list is here.