It's true that most people work 9-17 in Sweden, but it isn't legally mandated. A lot of the details about when and how we work is decided through collective bargaining between businesses and unions.
This system has very deep historical roots, and is something that the social democrats (that dominated swedish politics for a long time) and the unions are very proud of. Because of this Sweden doesn't have a minimum wage, for instance. It could be argued that this is a more liberal (in the classical sense, not the us meaning) way to go than what most of the US has done, which is a bit interesting.
(We do have laws about paternity leave, vacation time etc, so it doesn't encompass everything.)
Denmark also has that system, but part of it is law in addition to tradition. It's essentially impossible to be a large employer and refuse to negotiate with a union— all large workplaces have labor agreements in place. So there isn't the whole American system of "unionization drives" where unions try to "unionize" a workplace and companies try to keep it "non-unionized"; working conditions are negotiated between the employer and a union by default, except in some small shops. In cases where no one union represents the employees (at my workplace, employees belong to several different unions, and some belong to none), one union is appointed to be the main negotiator.
The union system is also much more highly coordinated, though part of that is indeed traditional rather than in law. Rather than every union having to negotiate its own conditions, some conditions are agreed at the central level, in framework agreements reached between the Confederation of Danish Employers (DA) and the Danish Confederation of Trade Unions (LO). The minimum wage is one of those: effectively all Danish workplaces have a minimum wage of 109 DKK/hr, the amount agreed to in the last framework negotiation.
That's funny, I have never had a job 9-17 in sweden during my 25 years of working life. First job was 6:48 - 16:00 (industry) or something odd like that. Next was 8:00-16:30.(university) Also worked 11-21 (store) and shorter other days at same job. Now I have pretty flexible times as long as I get together 40 hours per week.
This system has very deep historical roots, and is something that the social democrats (that dominated swedish politics for a long time) and the unions are very proud of. Because of this Sweden doesn't have a minimum wage, for instance. It could be argued that this is a more liberal (in the classical sense, not the us meaning) way to go than what most of the US has done, which is a bit interesting.
(We do have laws about paternity leave, vacation time etc, so it doesn't encompass everything.)