My group has played this 5 times and are becoming frustrated at the lack of combat. It seems that we all usually build up on respective frontiers and no one has the guts to commit. Either the person that loses that battle can't recover and gets steamrolled by the orb players or both parties have whittled each other down that they both get steamrolled and can't recover. The amount time and resources it takes to field your navy prevents us from being aggressive militarily. While I understand that not all races are designed to win through might, there most certainly are races that benefit from that type of gameplay. What happens in your groups?

I find that more but smaller fleets present more opportunities for combat. Sure, I love epic space fleet battles as much as the next sci-fi nerd but I don't really see it as the only possible way to resolve a military conflict. Besides, a smaller fleet is cheaper and faster to rebuild.

Build different size fleets, play aggressive races, make military alliances, tech for military advantage.

Everyone plays the game differently, it's one of the strengths of the game.

If your group tends to turtle then you need to be more aggressive, both on the board and with your diplomacy.

What optional rules are you using?

I have found that using the "artifact" rules strongly encourages combat, while not adding much complexity. Also, there are some quick setup map patterns in the player resources section

https://www.fantasyflightgames.com/en/products/twilight-imperium-3rd-edition/

that put the high value systems in the middle, encouraging players to interact to get the valuable systems. You could also take out the secret objectives that don't deal with Mecatol Rex, if everyone has a valuable objective that focuses on controlling it they should fight over it more.

I think the purpose of the white strategy cards was also to make it easier to fight. You might try using those.

I concur with the other responses. One of the prevailing design trends in the Twilight Imperium editions has been to discourage "turtling" (ie: sitting in your home systems building up a massive force.) This is difficult to do efficiently, and that's on purpose, because stalling the game to build up a huge armada makes an already ridiculously long game into an interminable one.

If you want more fighting without sacrificing your chances of winning, you need to focus on strategies that use many smaller fleets rather than one big one. That way, losing a battle doesn't cripple your ability to win the game. Also, keep in mind that battle, in and of itself, is not the goal. Fight as many border skirmishes with your neighbour as you like, but don't throw away all your ships and thus deny yourself the ability to score VPs.

I'd also second the idea of using Artifacts, if you aren't already. They greatly encourage combat in groups that are so inclined.

What size game are we talking? I commonly play a 3 player game and it can suffer horribly from the A attacks player B and then C takes advantage to attack one or both of the weakened combatants. I find that the battle lines can become static with huge forces but you can break the stagnation through advantages like key military techs, warfare strategy card or even the diplomacy card. Try attacking a hex and taking control of it, when he counter attacks retreat to preserve your forces. You can also strike early just to disrupt him before his borders are set in stone.

As mentioned before you should really have a clear goal in attacking a player rather than mere conquest, which can be incredibly resource intensive and usually does little to actually win the game. Also, it is easy to spread too thin with long supply lines, and the enemy replacing his entire fleet he just lost that round and placing the replacements adjacent to your now battered expeditionary fleet.

As Clausewitz said, "war is diplomacy by other means"

Do you have Shards of the Throne? You might want to try one of the Lazax prequel scenarios. I know there are treaty requests that involve getting another player to attack one of your neighbors. Also using the Distant Suns for Space and Planets does encourage aggressive plays with rewards of a free ship which can tip the balance and make combat seem more favorable.