But honestly, the main reason is that you can find PHP developers everywhere and they are cheap. Given that you want to build a project that grows and advances, that is maintainable, for which you require a vast ecosystem to kick-start your ideas and a lot of very good tooling and free sources of knowledge, you won't find a better platform than PHP.
Ok, Perl might be an alternatively hugely developed language but it is really complicated to find new developers for it. In contrast, you will find a lot of developers, testers, sysops that have the knowledge for working and administering the PHP tech stack. Yes, you could use the coolest language but how would you be able to pay super expensive Go/Kotlin/Node developers once your company has left start-up mode and is there really any gain from that? The risk to be betting on a dying horse might be too high or, less dramatic, everyone is just cooking with water.
If you want to earn money from your company and not just sell your expensive personell at some point it makes sense to start easy and advance the people you have. If you are building API based systems you are not locking yourself in a technology anyways. Given how flexible and fast-moving technology is today one could probably also find a good argument that you absolutely should always base your projects on PHP.