Because they don't have them, at least when it comes to deaths of people who have been found fit for work, they aren't on benefits anymore in a lot of cases so the DWP don't know whether they are still alive or not. They could probably get some kind of estimate, by trying to match benefit records with HMRC, or death registrations, as well as ex-ESA new JSA customers who've died, but it would be very noisy data and not the kind of thing that could be put together in an afternoon.
This whole FOI battle about this has all been a bit of a distraction, and not a very helpful one as this thread shows. Unfortunately dickhead is right, there is no evidence that tens of thousands of people have died, it's just not true, if it was it would show up in all sorts of figures, suicide rates*, cancer survival stats etc. All the figures the DWP are refusing to release are the ones which show the annual number of deaths of ESA claimants in both the Support group and the Work Related Activity Group (WRAG). The total number of annual ESA deaths has remained fairly constant, at around 50,000 a year. As some of these people had terminal conditions this isn't very suprising, but it has been reported or implied in places that all these people died because of welfare reforms.
What we do know is that in one year (2011 - the last for which we have figures) around 1000 people died who were placed in the WRAG - meaning they did not lose benefits but do face conditionality meaning the possibility of workfare or sanctions, being sent on shitty A4e training courses etc. That the death rate of people in the WRAG group is significantly higher than the general population, what this suggests is that some people are being placed in this group who are not really ready to look for, or prepare for work, and are certainly not faking it as media reports have implied.
We also know that about 30 people have died whilst in the process of appealing a fit for work decision, and there have been around 100 deaths reported in the media that were in some way linked to loss of benefits due to reforms. In some of these cases coronors have stated that benefit withdrawal was a factor in the deaths.
I find it a little depressing that people feel the need to say thousands of people have died to try and make an arguments against the reforms. One death is enough for fucks sake, and there have been plenty more than that - alongside which every measure of homelessness is rising and the use of foodbanks has soared. Hundreds of thousands of people have had their lives devastated by sanctions, Atos, the bedroom tax etc - we shouldn't need to invent ghosts, the suffering is fucking real and easily avoidable. And that suffering is built in to the now punitive nature of the benefits system, the DWP rulebook says it expects that benefit sanctions will damage the health of a normal healthy adult.
*there was a spike in male middle aged suicides in the most recent figures, this is worth keeping an eye on.