Let's say you are an auto mechanic. Would you not provide advice to your sister before she buys some car you know will bring her trouble?

Let's use Litecoin as an example. Litecoin is not protected by hashrate that consumes 15 GW of power, like bitcoin does. Could a state actor do a 51% attack against bitcoin? Very unlikely due to the enormous effort it would require but it is possible if they were so inclined. Could a state actor do a 51% attack against LItecoin? Still prettty unlikely, but mostly because there's little incentive to do so .. not because the effort is beyond that of a single nation state.

And litecoin has the vast majority of the hashrate for its hardware class (Scrypt ASICs). Now ask the same for the second most popular Scrypt-based altcoin. Or third-most, etc. Being proof-of-work does not just magically give protection.

So one reason for being vocally against other proof-of-work coins is to protect innocents who don't understand the differences between those coins and bitcoin.

Or take Monero. It wasn't that long ago there was a hard fork requiring an update by each node for it to continue working. That's a level of centralization, to begin with. You might feel the trade-off is justified, but bitcoin doesn't really force users to make such decisions.

Another reason is simply, ... all altcoins and tokens see bitcoin a a competitor. And vice-versa. Bitcoin would likely be much further along if it weren't fighting shitcoiners all day every day. Being "less shitty" than other piles of shit does not make that plate more palatable.