What is the reason why Buddhist monks don't work to support themselves? Do the monastic rules prohibit them from working?

It may be hard to find a comparative monastic tradition in the west although there are some monastic in Christianity currently most are really assimilated and intertwine with the Church which bears its existence in charity; so it's not practical to compare these with Buddhist monastic tradition. However, if we go back in history there were monastics in Christianity completely detached from the church, for example, the desert fathers ( Christian ascetics in Egypt and Syria ) were completely detached from the church, but following the Apostel's command that 'a person unwilling to work should not eat' they lived by the labour of their hand.
So, the question, in short, is why do Buddhist monks rely on alms for sustenance?
I don't mean to sound sarcastic, but for many observers, it's perplexing to see a young and strong monk asking for sustenance from old and ragged and gray women.

Gharāvāsa (layman) = ghara (house) + vasa (living) = the person who live in the house with 5 kāmaguṇa, the cause of disturbance, such as color, sound, smell, taste, and touch. And if you even read M.N. Mūlapaṇṇāsaka mahādukkhakkhandhasutta, 5 kāmaguṇa also included the lay-style-job for sustenance, too.

Samaṇa (monk) = sama (peace) + ṇa (person) = the person who live in peace, freedom from 5 kāmaguṇa.

Reasons for laying down the course of training for monks; Purposes of monastic legislation (vinaya):

“Because of this, monks, I will lay down a training rule for the monks for the following ten reasons:

A. For the advantage of saṅgha:

1. for the comfort of the excellence of the unanimous Order.
2. for the comfort of the Order.

B. For each monk's personal advantage:

3. for the control of shameless persons.
4. for the living in comfort of well-behaved monks.

C. To be the protector:

5. for the restraint of the cankers in the present; for the prevention of temporal decay and troubles.
6. for warding off the cankers in the hereafter; for protection against spiritual decay and troubles.

D. For the faith of the community:

7. for the confidence of those who have not yet gained confidence.
8. for the increase of the confidence of the confident.

E. For the advantage of buddha's teaching:

9. for the lastingness of the true doctrine.
10. for the support of the discipline.

So, buddha made the rule such as money-getting-disallowing rule for the monks.