It's not; it has practical consequences. Everything in a phone runs at a spectacularly low voltage compared to most electronics because the battery only has one cell. At small sizes it's simply not practical to make more cells.
Smart phones typically run lower than 3.7V so it's still not a big deal. The weight is a possible show stopper for cars but probably not smart phones (where size trumps weight) and of course they may be able to get the weight down by some kind of chemistry or materials magic (glass foam?)
Phones turn off automatically if the battery dips below 3 V, and this battery tops out at 2.7 V. It's totally possible to make it work, it just isn't terribly simple.
As for chemistry and glass foam, the voltage of a cell is determined by the electronegativity of the reaction in the battery. With lithium, it can never be very much above 5 V- atoms just don't pull on each other that hard. This already uses lithium, and the chemistry won't be able to improve much bast 2.5 V.
For any given chemistry, you can't increase the voltage without putting cells in series, because the atoms decide the voltage. Any tweaks to construction, electrolyte etc. can only increase the current, by giving it more places to flow.
"It's totally possible to make it work, it just isn't terribly simple."
Please, it's as simple as a three-component boost converter - one IC and two resistors. Same thing I use to bump 1.2V up to 3.6Vto run LEDs. It takes zero space (7x7mm total.)
That's 4 mm x 4 mm (more with passives, but not too bad), and hits a solid 95% efficiency at the normal load of an iphone, and is above 90% all peak loads. Heat dissipation will be fine, all parts cost about $1.20. Low EMI.
However, its still a dozen new components. It's a ~10% tradeoff somewhere and while that may be totally worth it, its not simple. Phones do not have square millimeters to spare.
Its like... adding a turbocharger to a car. Yes, it is possible. There are even kits, after a fashion. But it is still not simple.
Also, its not even at the stage of a hypothetical question yet. This battery would last about two weeks in an iphone under these load conditions. The only reason it "looks" possible is if you're considering a battery that is not this one.
If it's a true boost converter, it's also going to need an inductor somewhere. If it's really just driving an LED, it could be a charge pump, which uses capacitors instead. But charge pumps probably can't handle the current level that a phone requires.
"If it's a true boost converter, it's also going to need an inductor somewhere."
We use resistors as inductors. Plenty of wound resistors in teeny-tiny packages exist, that's how we have metal detectors with credit card-sized boards, the literal bulk of the unit is the frame and coil assembly and board housing + adjustment controls.
Older gas charge pumps can handle it, but they're bulky and they are lossy and generate a lot of heat.
Modern boost converters have efficiency from 80% to 95%. They also operate at high frequencies – 1MHz or even 3MHz, which allows to make inductors incredibly small. Boost converters aren't a problem for a long time now.
Remember that '3.7V' nominal cells range from 4.2V down to a low voltage cutoff about 2.8V (and in practice will not be let too close to that DoD). So stacking two of these '2.5V' cells will probably give a 6-4V usable range.
Remember also that the electronics is in three groups: 1.8V (most of the processor), 3.3V (peripherals), and raw battery voltage (RF front end output power amplifier). The former two will already be driven by switchmode regulators and the latter is much more flexible.
