Well, I guess the first thing to say is that lots of them do accommodate some level of part time working, normally 4 days a week or so, sometimes with additional days from home, a few job shares. Where it is often hard to accommodate is at trainee and junior level.
It is, unfortunately, the nature of the job at that stage generally - picking up the dogs body jobs as and when they come up, not a pre-planned work load. Arguably you could job share as a trainee, since less relies on ongoing intimate knowledge of the case/transaction/issue. However, that would be very hard to arrange given that trainee positions are generally filled years in advance, and the two job share partners (even if they could be found) wouldn't necessarily want to qualify into the same area - leaving them up in the air at the end of training.
But in response to your general point, I think the idea of 'teams' doesn't necessarily get you very far unfortunately. As I have said above, in my area of specialism, the 'teams' issue doesn't really apply. But even where it does, people often have reasonably distinct roles. Take a medium company sale, one corporate lawyer might be negotiating the SPA, another co-ordinating DD and the due diligence report. They know roughly what is going on, but not to the very high level of detail that would be required to seamlessly transition from person to person (as anyone who has ever covered a transaction from a colleague on holiday can vouch for, even the most thorough briefing never seems to cover everything that comes up whilst they are off). In order to run 'shifts' you would need to have a far, far greater level of knowledge about the other shift's work than team members currently do in a transaction. Effectively, you would be talking about job shares on a grand scale. And that would require a lot of time cross briefing (i.e. overlap between shifts), plus the additional salaries of paying two teams (ok each team would be paid less, but not half. And some costs, like health insurance, don't vary with hours). That would have a massive associated cost and I can't see one law firm taking the step to do that and take a huge hit in profits. Plus, clients would be very suspicious about 'paying twice' (an issue that seems to come up with frustrating regularity anyway - 'why do you need to have internal meetings', etc).
You know the one thing that could instantly change the culture in law firms and other long hours jobs - requiring that every working hour over, say, 40 or 45 be paid at an overtime rate. That would mean that there was a real incremental cost to working people too hard, and and incentive to use their time more efficiently. But let's be honest, it's a real 'top 2% of the first world' problem, so I don't see it being on anyone's agenda.
I personally think that the lack of female partners in law firms has a lot to do with the partner track - many women are expected to push things to the next level at just the time they don't want to. A less linear career progression would really help. I also think that chargeable units is a big issue - you can't work more efficiently and make it pay when your appraisal will be based on how many units you have recorded.