What's the deal with distributed SYN DOS attacks

I run some boutique web servers (Apache on macOS, https on 443) and most of them were being weird - occasional slowdowns. Apache logs were mostly blank. CPU and RAM were close to 0% usage.

But even a curl test on localhost was showing 1,2, or more seconds longer slowdown.s

After a lot of digging, realized that I was victim of some sort of DOS attack, which appeared to be a SYN flood attack.

In a normal SYN flood attack the SYN packets are sent from one IP address then never reply, leaving the server in a state with multiple connections stuck in the "SYN_RECVD" state.

In this attack, the SYN packets are actually being "sent" from multiple IPs - and one theory is that these are forged IPs, so the attack is really a reflection attack, where the attacker sends a forged IP SYN to my server, which replies (several times with SYN ACK)

I blocked the IP block /16 at the firewall and all was well, but this made me wonder:

How are modern OS's so vulnerable to this? On my macOS server, I could run netstat -anp tcp | grep '\.443 ' and see about 128 entries stuck in "SYN_RCVD" at which point the server just went to pieces.

In other words, if my server received 128 "SYN" packets, it would die for about 75 seconds.

Is this a macOS only problem? Are other OS's susceptible to this?

for this question, please consider "Use CloudFlare" off limits. I'm more interested in why modern OSs can be so fragile to a few (forged) TCP packets.