tl;dr version –
Because they are useless, greedy and don’t care about your need and your startup.
I know many “top” job consultants and head hunters who are “specialists” in finding executives (read as searching naukri/monster and making few calls, that’s it!) and as a startup, this is the last thing you want, to let someone else build the team for you. These consultants keep their own database and keeps on pushing the same names to all the companies without even looking at what the startup is looking for.
Also their primary focus is only on high end experienced clients from big MNCs, the 12+ lakh jobs so they can pocket commission of 10%. Personally I feels startups should avoid them like plague (speaking from personal experience as we tried to use such consultants for workmonk too), though if you know someone who is working in such consultancy and is your friend, you can ask that person to help you in finding such profiles for referral fees of Rs 5k or 10k.