Tar is seekable in the literal sense, you have to seek around until you find the file you want. You have to look at each header, if it's not the file you want y

Tar is seekable in the literal sense, you have to seek around until you find the file you want. You have to look at each header, if it's not the file you want you skip over that one file and read the next header. Because you only know how long that one file is. So you cannot even know where the third file is without having processed the headers of the previous two. It's "read, skip, read, skip, read skip" until you maybe find the file you wanted. ZIP otoh has the central directory index where you can look things up much faster.