Why curl does not send `Connection: Keep-Alive` in the header by default?

Here is the netcat output of curl.

```
$ nc -l -p 9000
HEAD / HTTP/1.1
Host: localhost:9000
User-Agent: curl/7.66.0
Accept: */*

$ curl -ss -iLI http://localhost:9000
```

Here is the netcat output of wget. Does anybody know why curl does not
use the same request header as the wget? Is there an option to make
curl behave the same as wget in this aspect? Thanks.

```
$ nc -l -p 9000
HEAD / HTTP/1.1
User-Agent: Wget/1.20.3 (darwin18.6.0)
Accept: */*
Accept-Encoding: identity
Host: localhost:9000
Connection: Keep-Alive

$ wget -q --spider -S -o- http://localhost:9000

-- 
Regards,
Peng
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