Why are non-zero non-absolute seeks not supported in python?

According to the python documentation,

• SEEK_SET or 0: seek from the start of the stream (the default); offset must either be a number returned by TextIOBase.tell(), or zero. Any other offset value produces undefined behaviour.
• SEEK_CUR or 1: “seek” to the current position; offset must be zero, which is a nooperation (all other values are unsupported).
• SEEK_END or 2: seek to the end of the stream; offset must be zero (all other values are unsupported).

For SEEK_END and SEEK_CUR, why is only 0 offset supported?
Has it simply not been implemented yet/does it need a PEP?
Is this by design? If so, why?

Is there a workaround with acceptable performance (for files with UTF-8, without using undefined behavior)?
All the answers on this question either use undefined behavior or don't handle files with UTF-8.