Technically, you're right. But in practice most LN nodes that process the bulk of payments in the network are online near 24/7

An offline node (with no watchtower) is also vulnerable to being cheated by peers.

Similarly, it's not technically required for an i2p relay to always be online. Of course, it will only be usable (by yourself or others) if its online.

The biggest difference between i2p and Tor is that you don't have to run ANY server to access Tor. Just download a browser (run a local SOCKS5 proxy) and you'll be able to route traffic thru the network without helping to route other's traffic yourself. It's easy to get started with Tor, but you're sharing resources (relays) with hundreds/thousands of other users which can lead to poor performance or denial of service.

"require running always online server" -- probably not the best way to phrase it. Rather, using LN and i2p (directly) require running a server and most people will probably run that server 24/7

It is technically possible for hundreds of people to use someone else's i2p relay (instead of each running their own relay). But that's not "the norm" like it is in Tor.