Because it is cheaper than your idea. Same standard, smaller hull, with no well-dock, no long-vehicle deck = cheap, better fuel efficiency, and more tolerable to attack even built to the same standard.
Most likely no, it isn't any cheapter, actually. Algeria paid its LHD-frigate an amount of money directly comparable to the target price for Type 31. And it included ammunition and training, both things that Type 31 doesn't really include as the first comes through other budget lines and the second through yet another.
Finally, we know what usually happens with target prices. Type 26 itself started out at 350 million, remember...?
Sorry, I think you are proposing to "arm Bay-class". Correct? (which is exactly what Algerian navy did). But, it has large hull and large vehicle-deck/well-dock = fuel in-efficient, easy to be hit, and very vulnerable. I am not yet convinced with this idea. What is the merit there?
The merits are:
- Where disaster rellief is the mission, it has space to carry people and stores and means to bring them ashore. Significant help, unlike what a frigate can carry.
- A greater flight deck and space for carrying multiple helicopters / UAVs to better respond to the mission. Counter-piracy to counter-smuggling to blockade of Libya or Somalia (the scenarios offered for a "war" use of Type 31, in other words) would all benefit from carrying boats, helicopters and Marines.
- Adds amphibious lift and greater flexibility of role.
- In task group ops, rather than adding a lillypad for a single helo, it can sustain more helicopters and leave space on the carrier for more jets.
- Once armed with a main gun and CAMM and related sensors, it has exactly the same weapons fit.
- Vulnerability. So, what? You are willing to load invaluable resources on the amphibs, with exactly the same level of survivability. Also, we have already been told that Type 31 will cut back in this area to achieve savings, so how much better will it actually be? It will also likely be a CODAD propulsion, probably not much on the sprinter side, so that we actually don't know how much actual "advantage" there might be.
Also again, the RN desperately needs to reassess what it really needs in terms of survivability when hit, and what can realistically be achieved in this area. They are paying a whole lot of money to protect the crew, and that is very noble, but any kind of damage that would sink a lower standard ship will mean, at the very least, a mission kill on the higher standard vessel.
You have to decide, very urgently, how many more hulls you are willing to lose in order to pursue a survivability that might save lives but that will ultimately still cost the war as you will run out of ships very, very quickly.
Also, passive survivability standards so far have effectively come at the expense of investment in decoys, active protection, sensors and weaponry. All of these are being cut back more and more, making it more likely that the ship will be hit in the first place.
Obsession with these mythical standards will end up meaning little. It already does on Type 45, which is horribly vulnerable to loss of power from battle damage (or simple failure) despite all the capital expended into separate machinery spaces and bla-bla-bla.
I also remain supremely unconvinced about these mythological RN "special" standards that supposedly go above and beyond what happens in other navies.