Some would say "Why is it unreasonable to just do the work you were hired to do", which has some truth to it - but it depends on how stringent you read "were hired to do". Most employees do many tasks that are not stated specifically in their job description and the job description often states that fact. In many roles, it's impossible to specify it all up front in detail since the nature of the work is dynamic and the employee has more of a broad role rather than a specific set of assigned tasks. Usually, there's also a tacit understanding the employee will himself seek out tasks that need to be done, help colleagues, get ideas for product improvements, etc. After all, they are getting paid and the money is only coming in because those tasks are being done. It would be unreasonable for an employee to do only the very basic things if there's a tacit understanding the job (and salary) is about more than what is exactly spelled out. That's probably one reason a contract often specifies a number of working hours. This means, just because you have done everything on your task list if you have only worked 10 hours that week your employer is free to give you other tasks within your area to fill up the work week. So there's an understanding about the total volume of work too even if it isn't fixed (in either direction). That's why it is in some sense meaningless when a worker says "Hey I will slack for a few days because I completed these tasks and it should be the results that count right?" because that can only be set IF the results are completely defined and measurable. So you could see the slack simply as a reflection that the employee was given too little work by the employer and now has a responsibility to report that to the employer and make his full work capacity available... it isn't a right to slack!
But counting _just_ hours is meaningless too - people work in widely different ways. Some people like to focus intensively and "work hard" but then leave office early and maybe even work less than they are supposed to. Others take a more relaxed attitude, running around the office small talking even up to a deadline, meaning they have to sit during the evening etc. There's also more inherent differences in productivity tied to experience etc. It's trivial to make a task take longer to complete. By having some variance in amount of time worked in practice, there's also an incentive for the employee to be relatively more productive outside what differences there may be in raw salary.