Because of standardization on "normal" workweeks, it's pretty hard to have the choice even if you're willing to forego salary for fewer hours. Google won't let you work a 4-day week for 80% of the regular salary, for example (or even for 60% of it!), and more traditional companies are even less likely to. You take their 5-day, 40-hour offer or you don't. You typically only have an option to work a 25-hour week if you have either a consulting/freelance career, or a very low-end hourly-wage job.
One other problem is that the whole 40 hour thing is also a farce in the presence of salaried positions - most places want you on site all day, under the assumption that they can keep you during crunch. If you are only working 4 days, unless you have infamous "come in on Sunday" talks with your boss, they have less opportunities to keep you for free overtime since you are salaried and they often put on weekly caps on extra hours before they have to pay overtime (or none at all, depends on the company).
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.
Google is actually known for encouraging their employees to work 80% of their work week on task, and the remaining 20% on creative pursuits of their own choosing, so in that sense I would argue that google is fostering creativity (which probably ends up benefiting the company overall) while being less demanding of their employees' time in a traditional sense.
