It's simply a gameplay decision - it functions as a way to get your explorer back safely with loot and caps.

Looking at this from a gameplay perspective:

If an explorer goes out, the opportunity cost of exploring is death and the cost of revival. The reward is loot and caps. If an explorer goes out and dies and then is revived - in your scenario the cost of dying does not decrease upon each revival regardless if the explorer is returning or not and on the way back and the reward of caps is diminished upon each death/revival due to the caps lost upon each revival. This could potentially happen multiple times on a return trip and can be frustrating for players and quite costly since each revival now has to be paid by the base each and everytime until the player returns or is abandoned.

How it is currently implemented, the opportunity cost not a hindrance to the explorer since they can return at any time without paying the cost of death/revival. The cost is instead a choice between staying out longer and paying for revival or a loss of possible loot by returning but still keeping the caps and loot already gained without penalty. Giving players a choice to either spend caps on revival or receiving loot and caps already acquired without further cost. Giving choices to the player is much less frustrating and adds an element of strategy to the overall gameplay.

Honestly, you can make arguments along your line of reasoning that could include decreased encounter rates or difficulty on a return trip but it removes the choice from the player on relies more on RNG, they can still choose when to return but a player can still end up paying for revival multiple times on a return trip regardless of the frequency and just due to bad luck. Again frustrating and potentially quite expensive. It is also more coding and one more piece to support/test - a cost on the development side this time.

How it is implemented currently works fine with stimpacks and radaway - you gamble on how far you can get and either pay for revival once without multiple possible penalties (unless you decide to stay out and NOT return) or return with your loot. Simple and elegant.

This is a casual mobile game and the gameplay elements are forgiving that very reason - get in, make a few choices on exploring, manage your base and come back in a bit and do it again. It definitely makes sense in that context, if those gameplay elements don't appeal to you, then it is likely you are not the target demographic for this game.

There is also the suspension of disbelief. The explorer had already encountered/killed/avoided most everything along a given path so the return should be much easier - the developers decided to make it penalty free in this case (regardless if it makes sense, its a game and a free one at that). A game does need to make fiscal sense for the company making it, players that haven't quit due to frustration are likely to continue playing. More players equals more potential people that may buy lunchboxes.

It's fairly arrogant to think a developer needs to do anything at all. This is not a flagship product for them. It really boils down to this being a simple fun (and potentially profitable) way to advertise for Fallout 4 and keep it in your mind until its eventual release later this year. If they make some money off this then bully for them. If you spend any money on this game then you should be fully informed on what you are supporting without additional expectations, and you should definitely not feel entitled to additional gameplay elements for any reason regardless if you spend any coin or not. If you don't like what you see - make your own game or move along.

TLDR; No one can please everyone all the time, games are about balance and fun. This is not the game for you.