As those people progress to writing code in other languages the number of improperly written applications in those other languages will increase. The issue is that so many people write PHP without having learnt how to write programs properly in the first place and as you say, these people without any idea of how to write proper programs then advise others in how they too can write insecure garbage.
Being able to code properly is independent of the programming language used. Unfortunately since the portion of the code to make it secure is generally around 75% of the total code and this part of the code is not necessary for working examples it usually gets left out of most books teaching programming. Experienced programmers know this and will add all the extra code needed but many newbies never get beyond writing example code never intended for a live environment.
PHP actually makes it relatively easy to implement most of the security needed. All you need to do is to avoid moving $_POST and $_GET values (etc) out of those arrays without validating the values first. That takes care of about 50% of the security issues that most newbie code has without even applying any security measures (since you need to do that to prevent junk input anyway).