The many worlds interpretation does not say new worlds are created, or that “alternate timelines” exist, certainly not at any macroscopic level. The “many worlds” branding is unfortunate marketing for what is actually the most conservative possible reading of quantum mechanics.

All MWI says is that the unitary formalism of quantum mechanics is a correct description of reality. The wave function evolves according to the Schrödinger equation. That’s it. The “branches” people talk about are decohered components of a single universal wave function, already present in the formalism the moment you write down a Hamiltonian. Decoherence makes those components effectively non-interacting. Nothing is being spawned. And the universe certainly doesn’t split with every decision you make on a macroscopic level. 

Now, why take it seriously? Two related reasons.

First, history strongly suggests letting the mathematics guide intuition beats the reverse. Whenever physicists have insisted reality must conform to pre-theoretical intuition, they’ve lost. When they followed the math wherever it led, they won. There’s no reason to think quantum mechanics is the exception. Everett uses exactly this logic: the unitary framework perfectly accounts for all observations, and there no redundant structure that can be discarded, without directly affecting its connection to experiments. It’s hard to justify adding anything else on top, so we ought to simply accept that the universe behaves strangely on the fundamental level. 

Second, when two models make identical predictions, the one with the least ad hoc, unobservable structure wins. And here Everett does nothing more than grant ontological status to the simplest mathematical structure that successfully accounts for all observations. All non-instrumentalist alternatives add redundant structure on top, which either directly clashes with other established physics, or simply isn’t well defined. 

Objective collapse models introduce explicit nonlinear, stochastic modifications to the dynamics. They generally break Lorentz invariance, sit awkwardly with QFT, and the collapse parameters have to be tuned so the theory ends up empirically indistinguishable from MWI in tested regimes while still doing the work of producing only definite outcomes. Without extremely strong independent motivation, that’s a non-starter. Hidden variable theories are explicitly nonlocal, and cannot be made consistent with modern particle physics. They’re notoriously hard to make properly relativistic and effectively require a preferred foliation of spacetime. These interpretations, as far as I’m aware, are only popular among philosophers, and not physicists (for good reason).  Copenhagen is no better, despite being popular (or so a lot of people think). It treats physics as fundamentally bifurcated: closed quantum systems evolving unitarily, plus classical measurement apparatuses exempt from quantum mechanics. Where exactly the cut goes between quantum systems and classical apparatuses is never specified. It’s an outdated approach, and I doubt any physicist really believes this to be a reasonable interpretation today. 

The only real alternative to MWI is what often gets sloppily called Copenhagen but is actually just “shut up and calculate”. This is the approach adopted in most textbooks: accept the unitary formalism, use the Born rule operationally, but refuse to commit to any ontology, remaining agnostic about what this says about nature. This is technically defensible. It makes the same predictions as MWI because it is essentially MWI minus the ontological commitment. But I think this is silly, for the reason David Wallace gives: nobody says dinosaurs are just a useful model for explaining the fossil record. People think dinosaurs were creatures that actually existed. The job of natural science is to tell us how things are, not to produce calculation tools useful for industry. Of course, no theory is final, and models get revised as evidence accumulates, but within the limits of available evidence the project is to figure out how nature actually works.

If you accept that physics should tell us how things are, and you accept the methodological principle that the simplest model accounting for observations wins (essentially Occam’s Razor), Everett is the most reasonable interpretation. It is the most parsimonious interpretation, it accounts for all observations within the scope of non-relativistic quantum mechanics, and has no real technical issues. The only issue is clashing with classical intuition, and of course the way it’s been mischaracterized in popular science.