You need to save an action for blocking.
You know yo are going first and you must decide if going all out, making more things without concern of your well being, or fight cautiously, holding an action for defense, or even be very defensive and hold both actions to defend from more than one attacker.
This assumes you know you are going to be attacked. Consider a character goes first, moves and fires at a target. Then the target moves and attacks him in melee. Do we assume it was sort of simultaneous and the shooter couldnt react? Did he have to anticipate he might be rushed and save an action, otherwise he is defenseless against it?
Same thing, there is a limited amount of stuff you can do in the limited amount of time you have in a turn.
If the character goes first, and there is no one around to worry about, and uses both actions to do a lot of stuff, while the character is doing the stuff (long and short actions or two short actions) someone comes and attack the character then the character is busy doing so much stuff that is unable to block. It may be because of a surprise or have the "hands full" with the stuff they are doing or simply is too focused doing stuff that didn't saw the attacker coming. End line is no block.
At least that is how I would game it as a GM.
In combat I would try to keep my players aware that if they use all their actions they can't react, at least until they have some experience with the game and combats.
Edit:
On the other hand if a character is alone (no enemies in sight) they may declare holding an action in case something comes up.
Narrative example: While the firefight is raging in the old warehouse you slip pass some crates, no enemy seems to have noticed you, you sneak along the wall trying to outflank the enemies, You finally move around and can attack the enemies from the side and out of cover. The character decides to make an attack, but instead of aim and attack does an attack and hold the fast action, possibly going back to cover in some crates because the player is not sure those are the only enemies and will be cautious. At that moment realizes someone just came from around and attacked with a knife.
If the player used aim and shoot would be unable to defend from the knife attacker. The character will be so focused shooting at the flanked enemies to notice the attacker.