Why do we say someone who has read many books is "widely read"?

Why do we say someone who has read many books is "widely read," or "well-read"? (Though the latter has a hyphen, and it could be called a separate word. Still, it has its etymology.)

Why didn't we just stick to saying "he has read widely"; why did we go from saying "he has read" to "he is read"?

In this case, the OED says read is an adjective, meaning precisely:

a. Of a person: experienced, versed, or informed in a subject by reading. Also read up (cf. to read up 2a at read v. Phrasal verbs). Only in predicative use.

This is the same adjective we find in well-read:

1574 J. Whitgift Def. Aunswere to Admon. 754 M. Doctor had beene so well read in the auncient Doctors.

1592 A. Day Eng. Secretorie ii. sig. T3, He ought‥to be well languaged, to be sufficiently red in Histories and Antiquities.

1632 P. Massinger Emperour of East iii. iv. sig. G2v, You are read in story; call to remembrance [etc.]

This usage of read is a bit younger than other uses of the past participle of "to read". The etymology notes for the adjective form say that it is derived from the past participle. The verb form has been around since Old English, while the adjective seems to have appeared some time in the 1500s.

This use of read, they note, is only used with modifying adverbs. So, in widely read, widely is an adverb that is modifying the adjective read. When an adverb modifies an adjective, it can often be in this "inversion". This is, for example, the order of "he is widely known". Another example is "I am really hungry" (and you wouldn't say "I am hungry really" with the same meaning).

Your terms are closely related, but not exactly the same:

He has read widely.

Here, "widely" modifies the verb "read"...

He is widely read.

Here, "widely" modifies the adjective "read". Here's another example that fits the second model:

He is badly injured.

To first the first model it would be:

He was injured badly.

The second model follows the rules for adjective order.