We use Present Perfect to talk about actions that is still in process. In this particular case "The have not counted yet" means "They are still counting". If they finished the process, we would use Past Simple: They counted it yesterday/ 2 hours ago/ last month etc.

yet expresses certain time from past and in the present, that or it is still up to in a present with an evidence showing it. Basically what you have to remember is 2 things 1) Yet is used with Present Perfect Negative/Question, is close to 'still'. 2) With seasons/weather if they are in progress so the season is going on right now you would use present perfect to talk about it's end. your sentence should be: "They haven't counted the statistics yet because the winter season (still) hasn't ended." or the last part could be "Because the winter season has yet to end" It won't end is incorrect because it will end. Recommend checking this link out: https://www.ef.com/wwen/english-resources/english-grammar/present-perfect-ever-never-already-yet/