Why monks have to take alms for their food?

I don't hink there is a canonical answer in the Pali-canon about the "why".

But in the books of Thich Nhat Hanh which I accept as quite authoritative there is an explanation in the following spirit (don't have the exact text at hand):
at one hand the monks shall not be concerned with the common daily life and samsara and what else and thus should -in priciple- should stay with the common samanas'/ascetics' practice. However, to avoid the danger that the monks become too remote from the society, from the human world(so to say) they should somehow remain in contact.

Derived from this that has two intertwined advantages: the monks give the people the chance to try and experience wideheartedness so they help them to train wholesome emotions, thoughts and practice, and even help them to simple stay aware of the existence of the Buddha's sangha, teachings and doctrine - and the monks themselves don't loose the contact with that from where they come from, with their own history and samsaric relations and don't become, for instance, hypocrites (the latter is a word from me, I don't really think that Thich Nhat Hanh had used this expression).

All in all, I find this a nice and very helpful interpretation - but as I said in the first sentence: I've no idea at the moment whether and where the Buddha might have been given such an explicite explanation of the "why" .

this might help. Maybe I should have posted my answer in comment section? I intend to jog memories and seek help from other readers. anyway, when Buddha returned to Kapilavastu after enlightenment and established Buddhism, his father king Suddhodana didnt invite Buddha to eat in the palace because he assumed Buddha would have come anyway, but instead, Buddha and his disciples took alms round on the streets. This brought a great embarrassment to king Suddhodana. Buddha took this opportunity to explain the importance of alms round, etc. Maybe someone can help me find reference for this sutta? First of all, alms round is a right occupation in the 8 fold path. Buddha said looking back for aeons, no one was ever ruined by giving alms.

Yes there is a clear reason why Lord Buddha himself didn't to that and forbid others to do it too.

As you know Prince "Siddhartha" (the prince who became our Lord Buddha) gave up all hopes of life and started his journey towards understanding. Let me tell you about how extreme that will was, Before giving up all food Lord Buddha ate only one "Mung" seed per day.

The reason why this rule was given has two sides....

If monks try to grow their own food and consume two questions arise. They will have to waste valuable hours to do the whole farming thing that they could use to further the path if there was no growing. The other reason is they will obviously be drawn to grow what they like to eat and that will not help their path (Because of it feeding the attachment to taste).

And every time a Monk consume temple grown food that would be a chance lost to a lay person to gain some good karma.

Historical updating of the rule....

Long after the Parinibbana of Lord Buddha kings took things a bit seriously. They saw that in case of a national emergency monks are at risk. Because if people have no food they can't give to monks.

So kings came up with a good solution. They granted temples and monasteries with land & people to work. So the villagers would build houses and farm and in return share the harvest with the temple. As kings have already given the rights to the land and all upon it the rule does not break.

Modern day updating of the rule....

Now as the society is getting busier there is a good chance that a family might miss their turn to offer food to the temple. So there is a trend to innovate. No matter if they are giving their offering or not people now bring some supplies with them to the temple as offerings, things like Rice,vegetables,Soya,Dried fish and etc. As the offering have been made it is OK to consume then when the need arise.

Monks can garden if the Land and all upon it has been offered to the temple. It happens all over Asia. But monks keep themselves from digging and such tasks because of the possibility of harming an animal. Most of the times all the work is outsourced to laypeople and a share of harvest is given.