I see that by default Cisco routers use tail drop as the congestion avoidance technique, which I believe can cause the tcp global synchronization issues.
Not sure if tail drop was chosen as a congestion avoidance technique.
Is it best practice to set routers to use random detect (WRED)?
Unless you're a QoS expert, I recommend against using WRED for its primary purpose. It's actually surprisingly difficult to get it to work optimally. That said, WRED can be handy if you want to manage tiered drops.
how com they don't come like this as default?
Principally, I believe, because any "fancy" QoS can load up the CPU. FIFO has the least overhead. FIFO is also much simpler to implement, one reason why you often don't find WRED an option on Cisco switches.
PS:
BTW, another candidate for a better default congestion avoidance technique, I would suggest WFQ - ah but wait, Cisco too may have at one time thought that as on serial interfaces, 2 Mbps and under, WFQ was the default over single queue FIFO.
