This isn't a question specific to any programming language. Say you have some file written on a big-endian machine, and you know this. If two single-byte values were written back-to-back, how would you know? Big-endian reverses the order of 16, 32, and 64 bit values, so how would you know you need to read it as individual bytes?
For instance, you write the byte 0x11, then the byte 0x22. The file then contains 0x1122. If you read that on a little endian machine, you'd have to convert it. So would you read it as 2211, or 1122? Would you know how?
Does this make any sense? I feel like I'm missing something super basic here.