1. Consumers are less tech-savvy, so having a single app as a starting point makes everything far easier than having to install many apps.
2. Brand is much more valuable. In emerging markets, with (overall) less regulation (and self-regulation) of markets, trust and brand carries a LOT of weight. If I trust company X with my payments, I'm also more likely to trust them with my transportation, my food delivery, etc (than having to verify/trust a new party). Having a trusted brand makes it very easy to expand into new verticals.
3. Regulatory clout. Once you have the scale (or political connections) to navigate regulation in one vertical, it's much easier to apply that to new verticals. You might "know the right people", know how to navigate the bureaucracy better... or in some cases, it's just easier for the government to trust you with a license than someone who's unknown to them.
4. Talent is more sparse, so clustering it in one place tends to make things more efficient.
5. Funding might be hard to come by, but existing companies either already have the cash or have connections to investors.
Overall, these things in combination just make it a lot easier for an existing company to launch a new vertical than for an upstart to do so. With time and as markets evolve, you'd probably expect more specialization to occur, but by then the super apps may already be entrenched enough to defend themselves.
For similar reasons, family-owned conglomerates tend to be very successful in emerging markets and span across a variety of unrelated industries. You just bought a place in a housing development built by a company owned by wealthy family X, then you go to the supermarket to buy some milk but you're not sure which brand to trust... Then you see the carton manufactured by another company from family X.