Note that the standardization you are talking about is also heavily driven by work regulations, saying that a work week is x hours and nothing else. If such restrictions were abolished I believe there would be much more flexibility (up and down) in the job market.
The regulations just put a maximum; there is nothing preventing companies from offering lower hours if they wish. Before those maximums were brought in, you had the same situation as now, just with more hours being standard: you had to agree to 6-days-a-week, 10+ hour days, or nothing. The union movement successfully negotiated that down to 5-days-a-week, 8-hour days as a cap, which was at least an improvement.
Note that there are drawbacks in several countries of not working "full time". You do not get the same rights towards a pension, you do not get the same rules for overtime, and other benefits do not apply. Therefore there is also a strong incentive, based on regulations, not to work less than 40 hours either.
