Because the offensive is ideological. The IMF is not drafting all of this from scratch, it is implementing one by one, all items from the Greek Board of Industrialists wishlist... Some of them really do believe that if they depress wages the economy will boom, because that is their idea of what a healthy economy looks like: permanently depressed wages, high inequality etc. On the other hand they are trying to gather in some taxes: Apart from making tax evasion laws stricter, Greece now has the highest proportion of indirect taxes as a percentage of total revenues at least in Europe. At the same time they are decreasing the corporate rate....
BTW this study is a look at the distributional aspects of tax evasion in Greece. While far from perfect (and possibly not tracking down money from corruption and crime or the real top of the income ladder) it does show that income underreporting is rife at the lowest (mainly undeclared wage earners, small shopkeepers etc I imagine) scales (where it has a very small taxable footprint) and at the very top (24% among the top 1% - that's beyond whatever they avoid legally) the largest part of middle income wage earners seem to be fairly honest. So contrary to myth, tax evasion in Greece is not ubiquitous but much concentrated at the top of the income ladder.

Note that they are destroying the tax base in more ways than one, yet they state it doesn't matter because they plan to cut expenses even more. Couple that with the planned sellout of everything but the kitchen sink and one begins to realize this isn't about improving the Greek economy but squeezing as much out of it as possible before it its remains are left to fester into a specially prepared second and a half world of former first world countries, with limited sovereignty.

The road of excess leads to the palace of wisdom - William Blake