They've found a way to dupe corporations into hiring them and also are riding the ubiquitous "outsourcing for lower cost cutting" trend of late, so no one really questions if it makes sense to hire them.
I've seen managers hire them a couple times - usually, it's often when the manager's department is very understaffed, incompetent and frankly a bit clueless (even as to what the strategic direction should be) [1]. Then, in comes the TCS salespeople, who convince the department head to effectively outsource the department to them, and they'll just execute all its functions, and provide strategic direction as well. They promise to implement "top industry standards", and, if you're say a head of department in a bank, it sort of makes sense to believe that these people have been to dozens of banks, and have collected a body of knowledge on what the best practices are. Also, I think it's extremely tempting for the department head, since he'll just have to deal with TCS managers from now on, and not run his department. Not to mention the cost savings. All in all, on paper, their offer is very enticing (also for the manager who makes the decision, as it looks like TCS will make his job much easier), and, given the outsourcing trend in the industry, it's not controversial, so managers very often go for it. Afterwards, the execution is always a disaster and huge disappointment (and manager's career is very likely seriously damaged) - but TCS makes its money in the process.
Oh, and on top of that, there's of course also just regular graft, if the manager is willing to take the bribes.
Basically, the likes of TCS are a parasite, exploiting systemic weaknesses of corporate management.
[1] The other common scenario is when the CEO/CFO demand x% cost cuts from the department and is basically suggesting outsourcing himself.
