One reason could be the revenue model, its hard to expand without recurring revenue streams. Many places, like Robert-Half, don't have coders on staff, so there

One reason could be the revenue model, its hard to expand without recurring revenue streams. Many places, like Robert-Half, don't have coders on staff, so there's no capital expenditures when the market is slow. A large firm would still have to pay people. Now, an example would be IBM, they're a huge development shop if you think about it, but they have many contacts and work on long projects that consistently bring in revenue. Its hard to get to that level, especially when your clients are diverse and hire you for one-off projects. In your example about law firms, I think its inaccurate to say there are no large nationwide firms. Its more accurate to say there are few large nationwide plaintiff's firms (although lawyers have other restrictions we won't get into). Plaintiff firm's clients don't usually come back for a second round, which makes it hard to employ many people who might just sit around doing nothing. Business law firms, on the other hand, do have large nationwide presences, because their work is often on retainer or consistent. I think you're right that the market is against the structure of a large firm. Its just much more economically useful to be small or be part of another company that also does other things, because the revenue stream is inconsistent and project based.