joined:Apr 1, 2016
my website has been actively present on the web for 10 years, my competitors are around since 1 or 2 years;
Who cares, Google certainly doesn't. The only people that really care about this are repeat customers, if you have enough of them the you don't really need to worry too much about what Google thinks.
my website template is professionally designed and I have renewed it this summer, my competitors use those free HTML templates
Is it possible that you overlooked something technical when you made changes (renew)? In my experience Google and users hate change and uniqueness, those free templates may look like crap but many sites are using them and so most users are used to them. Google likes that. Confirmation bias.
my website has content for each page, and it's focused on one keyword,
The concept of keyword is all but dead. There is no such thing as one keyword one result. Content should be written such that it responds to a customer/user's need not written to feature a "keyword". While your competitors may in fact be doing things worse, that is focusing on keyword variations eg: widget, blue widget, big blue widget, widget that are big and blue etc... but in doing this they may in fact be including some content that Google feels is a better match to the user's intent.
my website has a limited number of pages...
It has to do with user intent. If you feel that there are sufficient users with the intent targeted by your website then great. Otherwise broadening your reach may bring in more users, but broaden too much and you may be bringing in more users but then be unable to get them to convert as the users are only seeking info and have no intent to buy.
my website gets updated once a week with a new exclusive product...
As I said above, Google hates change for the sake of change. Obviously you must keep your content up to date and relevant, but if you are releasing products just to say to for the sake of change, I doubt that this will help you.
The biggest problem you have....
so I was checking out my competitors on SEMRush,...
As I have said above, the assumption of one keyword == one SERP does not hold. It is impossible for services such as SEMRush to account for this, as their service relies on that assumption. What this means is that while you are facing a downturn in traffic in business it is also possible that your competitors may be facing the same fate. Your competitor may be gaining all the traffic but then may be unable to convert the users. The net result is you loose, your competitor looses and the user looses, possibly, Google comes out the winner as the user resorts to ads to fill his needs (even that isn't certain, user may go to FB or Amazon)