If someone's renting an apartment, it's by definition not "their" apartment but the landlord's apartment. The landlord decides to rent to a particular tenant based on things like background checks, references, interviews, etc. These kind of assurances are not available for random people that the tenant rents to. Unless the tenant bears complete legal and financial liability for anything bad his subtenant does, it's asking the landlord to take on a significant risk.
Also, if each tenant in a large building could rent their apartment to up to 8 separate groups of people, you would have a constant flow of strangers through the building. Since the short-term renters don't have any stake in keeping the building clean or safe or quiet, this could pose problems for the other tenants. I've lived in apartments where even the long-term tenants left garbage in the halls and played loud music at 2am. I can imagine that someone who is staying in the building for a weekend might be even more anti-social, since there would be no personal consequences to their bad behavior.