What you're calling blitzing really is the game of fighting for territory from the outset then, which very much is the essence of the game. More territory means more metal extractors, which means more income from which, if you need to, you can build the heavy units of the late game.
A 1v1 or small team game usually plays out in several phases (in any of which victory is possible).
The first stage is the raider phase. This cannot be other than an open, fluid phase because nobody has staked out any territorial claims yet (as the game has just started). It's about scouting your opponent, keeping him away from your vulnerable areas and only then about killing units.
There are actually three resources in this game, metal, energy and as CrazyEddie
pointed out, time. A kill is only really valuable if it costs your opponent time to rectify. If I destroy your early expansion constructor, you not only lose the metal you invested in it, but also the time it takes to build another one and get it back out doing the job the original con was supposed to be doing. This is how raiders give me an advantage to press home.
By its end, the raider phase will have decided who holds what territory (and so who can start exploiting it). It might be the case that, due to a massive difference in skill and/or dumb luck (e.g. B737550 2 on Trojan Hills v05
) one side gains such a big advantage that they can quickly end the game. Most of the time, neither side will have such a decisive advantage and the game will move on to the mid-game.
In the mid game, each side consolidates control over their outlying territory, which might include starting to build up more serious fortifications. Their economies will now be able to sustain production of heavier, more expensive units. With these, a more attritional game will start, where both sides probe for weak points to exploit (that weak point might even be their field army, depending on your respective compositions). Every mex you take provides more fuel for your war machine until eventually, you reach a tipping point and your opponent can no longer sustain a fight against you. At that point victory is just a matter of time (and trying not to do something stupid that throws the game).
You reach the late game if neither side can gain that attritional advantage. Your success in holding ground to this point provides the foundation on which to build your end game play. You should have linked up your holdings in an overdrive grid which is perhaps benefitting from the vast energy output of advanced geothermal or singularity reactors. With your advanced late game economy, you can create a vast field army, build that nuke silo or even a Zenith or Disco.
It is quite rare in small games for the late game to be reached. Usually one side makes a fatal mistake before then.
You do see it more in large team games (8v8+) which also tend to do away with the raider phase simply because they're more crowded, which means its harder to cost effectively deprive your opponents of time - you might take out one guy's constructor, but if the guy next to him has one nearby that survives, it can continue the interrupted work much sooner, and of course it's much easier to have evenly matched sides with a larger pool of players.
I've talked in pretty abstract terms so far, but consider these thoughts in particular:
1) Defences are not there to protect you. If you need substantial fortification in your base, you've probably already lost.
1 a) Yes, that goes for chicken defence too - your defences should be creating a killing field from which you can harvest resources.
1 b) Partial exception: if, on account of having scouted your opponent you've learned they're playing scythes more investment is justified. The point of scythes is to stab you in the back, so make your back uninviting - but don't go overboard. You still need a front.
2) Defences are there to enforce your claim to territory. Build them to control choke points, deter raiders from outlying resources or to tip the balance of power at the front in your favour.
3) Your goal in game isn't to build a Detriment or a big base. Your goal is victory. You and your opponent have exactly the same starting choices and resources. You must deprive them of theirs while expanding yours.
4) "Rushing after the other guy" is a massive over simplification. The raiding phase is the most tactical part of the game. You have limited resources, limited information about your opponent and need to make tough choices on how best to use them.