Here are some possible rationalisations for such companies existing, such companies betting on acquisition by a limited number of buyers & other concerns:
- Smaller companies can sometimes do things better.
- Smaller companies can sometimes do things cheaper.
- Small companies can fail, so they can try riskier things.
- Small companies may not have as much of a brand to protect, so they can try riskier things.
- More features can be put to market test by multiple indipendant companies then by the 'parent' alone. Individual features can be put to market test and good ones have a better chance of being discovered.
- Features can be tried multiple times by multiple companies, with the best one ultimately emerging.
- Startups (using a certain definition) have a 1/x chance of succeeding at launch. Success/failure is binary. The risk of faling completely is extremely high. Therefore, a 10% or 20 or 40% chance that Twitter will disappear while they rely on it to survive is not as scary as it is for an established company. If your chances of success are only 1/5 anyway, going to 1/6 is not too bad. Basically, startups are structured to handle that sort of risk.
These reasons may or may not add up to a net efficiency gain overall, better features or whatever. Redefining what companies do themselves vs what they outsource is an interesting part of how economies evolve. Outsourcing innovation (even if it is just a small feature) might be a good move, sometimes. The market gets to decide.