There are many different reasons for why people are sending you trades for those pets.
Although, yes everyone is able to get these pets, some people:
-Are missing certain outcomes, so they will trade for what they are missing.
-Want only a specific outcome, so instead of taking chances with tokens, they'll just trade for a specific.
-Hoard these pets and are trying to get more to add to their collection.
-Might be trying to scam you by giving you pets of a lower rarity and are saying they'll be worth the same when that's never guaranteed.
-Just like the pets and want to get more.
-They could be stocking up on extras for other newcomers later in the year (Some people try to get doubles from people so they can gift newbies with pets no longer available.)
There's really no way of telling what the person's intentions are, but you can always ask or just cancel the trades if you would like.