The main difference I can think of is that unlike films, which are discreet projects with hard beginning and end dates, software projects never really end. Maintenance can go on indefinitely and usually the most knowledgeable people to do that maintenance are the people that built the project in the first place. That makes some proportion of people likely to stay with a project for a longer time than it takes to just code up the requirements and generally makes turnover cycles less predictable than they are for people working on films. With less predictable turnover, agents (who generally make money at the time a transaction completes rather than continuously) would have less predictable income streams so they are less incentivized to do it. Also, even in movies, from what I saw, outside of top talent who command large contracts, all the other folks didn't seem to have agents. Thats probably because the transaction amounts for a given contract don't make sense for either party to participate. All the grips, electrical people, PAs, costuming, craft services etc workers were finding work just as a software contractor might -- through connections from friends, colleagues, and people they worked with on previous projects. Many are also part of unions for their respective part of the business so I would expect they get some assistance in finding projects from that as well (e.g. if there is a union production in town they are usually required to hire only people part of the various unions -- so if you're one of the only union members in a region you could get work that way).
I don't think agents are totally incompatible with the software industry, but I do think it would take a somewhat rare combination of highly paid project with a discreet, somewhat consistent term of employment (maybe coding up financial some kind of financial model or data pipeline for a hedge fund would fall under this?) to make it worthwhile for agents to specialize in.