and only have different pronunciations?
It's probably not easy to answer exactly why this happened (past and present being identical in spelling), because I don't think anyone ever set out to do things this way. Like most standard English words, read was not always spelled this way. The OED lists, for example, rædde, redis, redys, reeds, reids, redds, reed, red, redd, etc. over the course of written English history. The OED does briefly touch on the past/present spelling issue:
The present-day spelling of the past tense and past participle forms is analogous to the present tense; the spelling red for past tense and past participle is found only in isolated instances in the modern period and was perhaps avoided because it coincided with the standard spelling of red adj.
To me, that is not a terribly satisfying answer, but it could be the correct one.