Many places in the Shaastras says the Physicians and musicians are not eligible to receive gift, considered as witness, invite to Shraddhas, etc. Why are these professions have been classed like this?
Example:
A palmist, a thief turned into a merchant, a fowler, a physician, an enemy, a friend, and a minstrel, these seven are incompetent as witness. [source]
One should never cultivate intimacy with physicians, or with children, or with persons that are old, or with one's servants. [source ]
Those Brahmanas that practise the calling of physicians, those that receive regular pay for worshipping the images of deities established by the rich, or live upon the service of the deities, those that are observant of vows from pride or other false motives, and those that sell Soma, do not deserve to be invited. [ source exclusion from Shraddhas]
Those Brahmanas that are, by profession, vocalists, or dancers or players or instrumental musicians, or reciters of sacred books, or warriors and athletes, do not deserve to be invited. [ source exclusion from Shraddhas]
He who takes food from a physician takes that which is no better than excrement [ source ]
If a Brāhmaṇa is a (professional) singer, what is given (to and) by him becomes fruitless. [ source ]