Because people are stupid. And stuck in the 90's. That's the main reason. Other reasons include self-hatred for spending so much time on RoR's/Node's penis. That's definitely the second leading reason. Making less money then "stupid PHP programmers" cause they seriously thought they could develop real applications after learning JavaScript. That's pretty high on the list.
Look the bottom line is PHP has some weird issues (naming inconsistencies, parameter ordering, security issues with deprecated functionality, etc. etc.) but you can find issues on the same level with ANY language. Professional PHP dev's can consistently build large enterprise applications that are readable, scalable, maintainable and performant. Anyone who tells you otherwise is simply not familiar with the transformation that PHP and the community has gone through since version 5.3.