I guess it depends on what your definition of endemic is. My understanding is that it is widespread, and there are many reasons for it.
> Second, if tax evasion is endemic, then why?
At the simplest level, tax collection is fraught. To the extent that it's easier to evade taxes, it will be evaded. This is not a cultural trait, it's a human one. Tax collecting organizations around the world have struggled with this issue since taxes were invented. The difference between what you think ought to be able to be collected and what you actually collect is called "The Tax Gap" ... some good stuff, if you google it.
My impression is that the tax collection aparatus in Greece is one of the easier ones to evade, especially by self-employed professionals. As an example of things that can be done to increase compliance, consider the US's system of withholding from pay checks. In order for you to evade income tax, it would be necessary for you to collude with your employer. Does it happen? Of course: many people get paid "under the table" ... but overall compliance with individual income taxes in the US is very high because of this structure. Similarly, corporate tax compliance is high for publically traded companies: the requirement to have outside auditors does a lot to keep many public companies compliant. Do some public companies take on tax evasion as a business line? Of course. Tax compliance is worst in the US -- just like Greece -- among the self-employed, with estimates of under-reporting ranging in the 30-40% range.
The IRS spends a decent amount of time thinking about the Tax Gap in the US.
http://www.irs.gov/newsroom/article/0,,id=158619,00.html
Another measure of "tax efficiency" is how much money is spent in order to collect a given dollar of taxes. The IRS manages to be the "most efficient" in the world in this regard, as they spend less than $0.50 for every $100 they collect. Anecdotally, I have been told that Germany spends up to $7 for each $100 collected, mostly because they do not -- or did not at the time I was told this? -- have compulsory withholding by employers.
/jordan