They are written poorly. They get the recipient's fields and sub-fields wrong, sometimes horribly wrong. They contain boilerplate in a peculiarly quaint English. They read eerily like the Nigerian scam mails -- except for the promise of a couple of million dollars.
Many professors think they are being spammed -- so they delete these mails without even bothering to read them.
Here's Paul Goldberg:
IIT student seeks internship at your esteemed institute
A question: has anyone taken on one of these prospective interns? (And if so, how was it?) Most of them would clearly be hopeless, but there are some that look like they might be OK. The trouble is, there's some kind of economic principle at work here, that says that in a market that's flooded with bad eggs, the good eggs cannot be sold. In this case, what happens in that we end up deleting all these emails without reading them.
Goldberg also points to an earlier discussion in another professor's blog.
While these posts are about internship requests from IIT students, there's also a large number of non-IIT students who have got into this habit of sending a form e-mail to hundreds of professors.
I am in materials engineering, a field that produces perhaps 1000 Indian graduates a year, and I work right here in India. And I feel I'm getting too many of these requests. I can only imagine the frustration of professors -- especially those in the US or the UK -- in computer science and mechanical engineering where the numbers are quite easily 50 - 200 times larger than in my field.
Thanks to Raghu for the pointer.