Reportedly this question was put at a cross-Atlantic economics conference in the memorable, if non-PC form, of "Are Europeans lazy or Americans crazy?"[Causa, 2008, page 5]. But it's more complex than that.
For a start, it's not all Europeans. The OECD reports that in 1999 total hours/worker in the USA, while above the OECD average, was lower than those in Poland, Hungary, Italy, the Czech Republic and Greece, and pretty close to those in Ireland. The exact distribution varies quite a bit from year to year but the USA is only really an outlier relative to north-western Europeans (loosely defined). And the USA is consistently well below hours worked in Mexico and Chile. Perhaps the question should be "Are Canadian and US citizens lazy, or Mexicans and Chileans crazy?" :)
Secondly, the conference question implies cultural explanations for the difference, but if we look at the long-term, the USA has only recently become an outlier in terms of working hours relative to North-Western Europeans. From figure 3.1 (page 25) in this ILO report on trends in working hours, back in 1870, people in France, Germany and the Netherlands were working much longer hours - over 3,000 a year than the United States, United Kingdom and Australia (where people were working less than 3000). Working hours for all 6 countries fell and converged in the 1920s, then diverged again during the Great Depression and WWII, before drawing together again in the 1970s. The USA only moved to the top of the working hour band in the 1980s (and saw an increase in working hours in the 1990s, though still very small compared to 1870s figures). So the short answer to your question is that the USA has not always had a workaholic culture compared to north-western European countries.
So what drove Americans (or at least ones from the USA, Chile and Mexico) to be crazy, or the north-western Europeans to be lazy?
The causes of differences in working hours between countries is highly debated, as my starting quote indicates, and there's been many efforts to explain differences. Factors used to explain it include employment regulations and policies (eg unemployment benefits, union power, wage bargaining systems, government regulations of contracts), government fiscal polices (taxes and government spending). A sucient summary is that:
Hours worked have fallen especially in continental European countries characterized by strong unions, extensive welfare coverage, high taxation and prevalence of social democratic governments. (Alesina, Gleaser, Sacerdote, 2005, page 24).
However, it is hard to disentangle the impact of these between countries, as they are not independently determined. For example, high marginal tax rates reduce the return to labour, and therefore might encourage workers and unions to lobby for more vacations and shorter working weeks rather than higher pay. Higher government spending on social benefits might also encourage more of an emphasis on leisure time rather than savings (though Americans are not strong savers). The political factors that drive increased unionisation might also drive increased government spending on benefits. Etc. Arguably also the high expected inflation of the 1970s and early 80s made unions and employers more inclined to agree on holidays in their bargains: high inflation is volatile inflation so the value of a wage settlement is hard to estimate while a day off is a day off. This would explain the timing of the divergence in working hours starting in the late '70s, early 80s, as shown in the ILO report.
Alesina, Gleaser, Sacerdote, 2005, argue that there's social multipliers at effect here: holidays may be more fun if your friends and family are also on holiday - they note that with the shift to a 5 day working week rather than a 6 day working week countries converged on people typically taking Saturdays off as well as Sundays, instead of Sundays and Mondays. Similarly, being able to take long holidays (eg 5-6 weeks in contracts) can be more attractive if your family and/or friends are free to go away.
In summary, a complex interplay of policies and politics appears to have driven a shift in some countries, particularly in north-western Europe, since the 1970s. This shift was probably increased by social multipler effects.
Whether Americans are crazy to have not copied this shift? I leave that one up to you.
A few notes:
I have here equated "workaholic" to "hours worked", I hope someone else has the knowledge and time to discuss potential more cultural interpretations, for example value placed on the idea of work, and how they might differ across countries.
The accuracy of data on working hours is a bit doubtful, partly due to differing concepts, and I'd advise not getting caught up in small variations from year to year, or between one country and another.
I mentioned the increase in US working hours in the 90s but I've not gone into it since the sources I have to hand tend to run riot through the 20 years rule.
Sources:
Lee, McCann and Messenger, Working Time Around the World: Trends in working hours, laws and policies in a global comparative perspective, Routledge Studies in the Modern World Economy, 2007, https://www.ilo.org/wcmsp5/groups/public/@dgreports/@dcomm/@publ/documents/publication/wcms_104895.pdf
Lorenzo Carbonari, Vincenzo Atella & Paola Samà (2018) Hours worked in selected OECD countries: an empirical assessment, International Review of Applied Economics, 32:4, 525-545, DOI: 10.1080/02692171.2017.1353593, https://www.tandfonline.com/doi/full/10.1080/02692171.2017.1353593
Causa, O. (2008), Explaining Differences in Hours Worked among OECD Countries: An empirical analysis, OECD Economics Department Working Papers, No. 596, OECD Publishing, Paris, https://doi.org/10.1787/244230044118.https://www.oecd-ilibrary.org/economics/explaining-differences-in-hours-worked-among-oecd-countries_244230044118?crawler=true
Alexander Bick, Bettina Brüggemann, Nicola Fuchs-Schündeln, Hours Worked in Europe and the US: New Data, New Answers, Discussion Paper No. 10179, August 2016, IZA http://ftp.iza.org/dp10179.pdf
Alberto Alesina, Edward Glaeser, Bruce Sacerdote, Work and leisure in the U.S. and Europe: Why so different?, 2005. https://scholar.harvard.edu/alesina/files/work_and_leisure_in_the_u.s._and_europe.pdf