As an accountant (of over 40 years) now with my own practice, I can see it from both sides. Trouble is that a lot of the work, such as audits, can involve full days, i.e. a client wants it all done and dusted within, say, a week, which requires full time working at their site. They don't want the disruption of someone taking two weeks on site only working 10-3. In fact, usually you have to work long days to finish within a week, i.e. starting a bit earlier and finishing a bit later. Some audits aren't particularly local to the office, so you also have travel time before/after the working day to factor in too!
The "part time" jobs have usually been in the tax departments, where "jobs" are shorter/smaller as you can usually do a tax return within an hour or two. My experience has been that tax departments are usually dominated by middle aged women to facilitate part time working, whereas audit departments are mostly younger people and middle aged men!
Currently, in my own practice, I find that most clients want meeting appointments either first thing in the morning or late afternoon, to fit in with their working commitments, often pre 9am or post 4pm, so again, if I only worked 10-3, I'd probably lose a lot of clients who would find meetings within those hours problematic.
As it is, I do work part time, but it's very flexible. Some days I start and finish early, i.e. 8 till 3, sometimes I work a few hours on Saturdays and Sundays, somedays I start late and finish late. Mostly to accommodate client needs, but if no particular client requirements, I am sometimes flexible for my own personal reasons, i.e. start and finish early so I can have the afternoon in the garden etc.
It just wouldn't work if I was inflexible and tried to enforce say a 10-3 working day. Clients would leave in droves as it wouldn't work for them.
This is the problem really. Firms have to basically work backwards from what works for the clients. If they have too many staff wanting to work part time fixed hours, then that puts the burden and pressure on the other staff who are full time or more flexible.