Because the difficulty is different
Of course, that "replay" is not "effortless". You have to mine a valid block header. But the problem (or the feature) is, that such transactions can be migrated. Which means, that even if someone will destroy testnet3, then exactly the same transactions, with exactly the same transaction IDs, can be replicated in testnet4, if there will be enough incentive to do that (and because some people started assigning value to testnet3 coins, then it may happen).
You can't take out a single block from the middle of that chain and place it on top of another chain.
The only needed thing is to match the coinbase transaction. If that matches, then the whole chain of transactions is also matching. In general, if you recreate all UTXOs, used as inputs in a given transaction, then it can be replayed. Which also shows us, that if we want to replay something, then we have to trace that, only to the nearest coinbase transaction, because everything before that is just unsigned, and sent as fees, which can be taken from any transactions. The only requirement is to have matching amounts.
For example block 1,234,567 in Testnet3 is referencing block 1,234,566 on Testnet3 if you take that and try to place it on top of for example Mainnet chain at 1,234,567 it is rejected because the 1,234,566 reference is not found.
Of course, but note that the coinbase transaction does not contain the previous block hash. It contains only the number of the block. Which means, that such transactions can be replayed between unrelated networks.
This chain also goes back all the way to a hardcoded Genesis block that is unique to that specific chain to prevent exactly what you explained.
Of course, but the chain of block headers is the only unique thing per network. The chain of transactions, and the chain of signatures, can always be replayed. For example: you send 50 BTC in the mainnet block 1,234,567 as fees, and you also send 50 tBTC in the testnet3 block as fees. And then, if you mine both blocks in both networks, then you can use the same coinbase transaction, with the same amount, and reach identical transaction IDs in both networks. And since then, you will have a connection, similar to the one existing between BTC and BCH, where you have a lot of shared history. But here, it goes further, because you don't have to start from the same point, to meet in the same transaction. Because if something is sent as fees, then starting transactions can be different in both networks, but the final outcome could lead to identical transaction data.
