I've never understood the "can't resource internally with perms" argument. Hire people to do the job, and then lay them off when the project is over. These projects often last for years and frequently go over budget.
How difficult expensive that is depends on the country you are in - not all countries have a fire-at-will type legal system.
Also having frequent rounds of firings impacts company culture - making it much more transactional ( everyone for themselves ) - some companies value their employees and culture and the collaboration and long term thinking that can bring.
One option that's increasingly common is hiring people on fix term contracts - however that does create a sort of divide.
Layoffs have a significant morale impact for the rest of the company that lasts months or even years afterwards. You should not hire FTEs you intend to fire.
