R/W speed iMAC HDD vs. external HDD

Recently bought iMAC 21 inch 2017 with 1TB HDD. Performed HDD speed test using Black magic, to compare used external USB 3.0 HDD Seagate (bought in 2016 or earlier). The results are as follows

1. iMAC HDD: Read speed about 96.5 MB/s and Write speed about 98.3 MB/s.

2. External HDD: Read speed about 109.1 MB/s and Write speed about 114.0 MB/s.

Question: why the external HDD via USB 3.0 is faster? Doest it mean using external HDD is better than the internal one? Why at all put a slow HDD inside and make it difficult for a user to replace it (screen tape removal, etc)? What it tells about the quality of Apple products?

Of course, I have an entry level product (but then would it be unfair to compare it with external HDD built some years before it?). The reason is also a quality of Apple products. Before I had iMAC 27 inch mid 2011, top notch with CPU 3.4, SDD inside, etc. It has burned its graphics card (famous AMD radeon). It has done it TWICE, second time after APPLE professionals replaced it in 2016 under the exchange program by (SUPPOSEDLY) new one. It lasted 1.5 years. Now this junk is non reparable at all. So this time I bought iMAC with Intel graphics card. Hope at least it will last. Now I have external SDD ripped from my 27 iMac taped to the back (again Apple does not think that their rather slow HDD need replacement).