Why do some websites have their hashes on the same area as the download link? [duplicate]

This question already has an answer here:

  • Does hashing a file from an unsigned website give a false sense of security? 11 answers

Some sites allow us to use hashes to make sure that the file we download is what it should be and that it is not tampered with.

Some sites may have their hashes on a different domain probably on a different server other than the one the download link is on, like the site for Ubuntu (This may prevent someone from hacking the site and then modifying both the download link and the hash to fool people.), but some sites like Kali Linux have the hashes on the same page as the downloads link.

Isn’t this bad? What prevents someone form modifying the hash if they can already modify the download link?

Input transaction hashes not matching transaction hashes

I parsed blockchain .dat files. I wanted to find transaction input address for a given transaction input.

For this I looked at “prev hashes” from inputs, reversed the byte order in them and then searched for the corresponding transaction. However, in most of the cases, reversed prev hash didn’t matched any transaction hash (actually about 5% of hashes matched only).

Is it possible or am I missing something here?

Sorry for the question but I am quite new to bitcoin.