You ask this question as if it's an established fact that SE does not have good coders.

Well, several "simple" questions:

How do you even tell they do not have good coders?
Did you have a peek at their code? What is their weaknesses?
How you define a coder as being "good enough"? Do you even do programming yourself?

Are you aware that "code" is just one part of a very huge and complex framework of software & hardware mingles together to make "game" happen?
Do you know that some part of this framework is totally out of SE control? Does SE has control over what PC/Network/Drivers/Firmware/ISP that you use?
Have you consider the fact that, any single bottleneck/weak-link in this framework, will impact your gameplay?

Whatever symptom that makes you think they do not have good coders, do you even consider/comprehend that it may be due to other reasons, instead of "code" itself? Perhaps hardware/network/third-party program/etc? For a good coder, how to cater for all these possibilities? Is there even such a way? If such a way exists, why wouldn't SE knows it? Why wouldn't the world know about it? How come it hasn't become the Holy Grail of programming, and practice by every coder by now?

Why blame the SE coder?
How many bugs/problem you encountered, that you are so sure it is the SE coder's fault, instead of the coder of one's ISP network modulator/router/firmware, or the coder of one's GPU/OS/virus-scan/third-party software?

Do you consider the fact that, only those with problems will complain? And that doesn't represent all others who play just fine?
Did you exclude those who caused the problem unto themselves, most probably totally unaware of it, such as using a crappy virus-scan software, or unaware that some auto-updating/downloading/background-tasks/malicious software/trojan/data-miner running?

Simply, why blame the coder?