Lawyers bill hourly. But here's the rub: most lawyer tasks take less than 2 days (the modal corporate legal task is probably a 4 hour long contract review).
That's not true of software development. Serious development projects are broken into major milestones weeks apart from each other. That's why the term "weekend project" is so evocative.
Also, lawyers command significantly higher hourly rates at almost any level than the equivalent level on any sort of IT/software consultant scale.
For one thing, hiring a lawyer is much like buying an insurance policy in many cases: the potential amount of money you save if something bad does happen is dramatically greater than the fee you pay. Take the usual arguments about business value generated by IT consultancy, and scale them up by some number of orders of magnitude.
It doesn't hurt them that they operate in a tightly regulated industry with high barriers to entry and effective immunity to market disruption. This supports a culture of high pricing where, regardless of whether it really does represent good value for money, clients expect decent lawyers to be expensive but bill by the hour or shorter.
As a curious aside, the norm in the legal profession is that if lawyers want to fight for a good cause they do that work pro bono and still charge a small fortune for their regular clients. It is almost unheard of for a lawyer to charge real money but at well below the normal market rate for their services. It's an interesting contrast to other creative industries, where spec work is often frowned upon and you have the option to outsource work to foreign businesses who will charge you peanuts as long as you can accept the quality of work that buys.
That's not true of software development. Serious development projects are broken into major milestones weeks apart from each other. That's why the term "weekend project" is so evocative.