I think because of following reasons,
- JMeter is server side load testing tool (means we are worried about server response and not about client i.e. browser)
- When we see resonse in browser, sequence is, raw text response is parsed first, css applied, any other styling features are applied, JS if any starts running and then you see a complete web page loaded but all this is client activity and not server
- When we are interested in, why response from server is slow? or what is the server response at specific load? then we use jmeter which will give time from request sent to server till we get first byte of response which is server response time + latency
- Thus Jmeter response time is less than browser and is more important in server side load testing
But there are plugins available which will help you simulate sending parallel requests and ajax requests to server, by default Jmeter sends requests sequentially but plugins and tweaks are available for your need.
I hope this clarified your doubt. :)