It's trivial to use TOR or I2p with most cryptocurrencies, so baking it in is not needed--I believe Monero is waiting for Kovri because of some concern with network security. Apparently baking-in TOR or I2p has an effect on Monero's network security that Kovri doesn't have--you will have to pm one of the Monero devs for an in-depth answer.
This is a
really common misconception, but it's not true. It is
not trivial to just use Tor/I2P with any cryptocurrency, if you care at all about your privacy and security. If you're not using onion addresses (or the I2P equivalent) then your traffic is traveling unencrypted across exit nodes, who can view and even censor your transactions. It's well known that some of these exit nodes are operated by governments with an interest in doing exactly that sort of thing.
The integration of Tor or I2P with a cryptocurrency requires thought and care, which is no doubt exactly why Monero is pursuing Kovri, and is also why Spectre integrates Tor and uses only onion addresses, rather than just pointing the SOCKS proxy at Tor like people suggest you can just do with any cryptocurrency.