First, there exists practically zero confessions or any other evidence that any "blood doping" occurred any other time than before big competitions in Finland. The transfusions of Finnish athletes were administered in hospitals as there was no university department of physiology where the operations were administered (unlike in Italy).
1) This increased the a risk of getting caught literally red-handed. At least some cross-country skiers were physically seen taking transfusions and on the case of Kaarlo Maaninka (1980), there was some inside information leak (possibly same thing).
2) Transfusing has always health hazards assosiated even when everything is done properly. The cost-benefit - analysis limits the instances when it is used. Why to target to run well in June if the risk is getting a hepatitis and possible screwing your possibilities in Olympics later in the year?
3) If blood doping was as easy as buying candies, then there is no reason why Viren wouldn't have blood doped during the years between olympics. Apparently he didn't.
In 1970s there were some studies that indicated that blood doping helped performance, but there was no consensus that it truly worked. From academic point of view, the flaw in the Swedish studies (1972, 1976) where positive effect was noticed, was that there was no control group, so there could've been other explanatory factors than that the underlying theory was sound. There were some sophisticated studies with control groups in 1976-1978 that didn't find any value of the method.
If Finns did research the subject, they did it mostly in secret (I have my own reasons to believe that this could've happened), as to this date, there has been only one Finnish study published on the issue (1977). The authors conclude that they "did not find any statistically significant improvement in the physical performance attributable to autotransfusions" and that "the training effect is much greater than the effect of blood transfusions even in well-trained men".
To close the debate on the "confessions", here is a dry and monotonic summary of the confessions and some other official knowledge.
1) Mikko Ala-Leppilampi. In 1981, admitted blood doping in 1972, 1973 and 1975.
2) Kaarlo Maaninka. In 1981, admitted blood doping in 1980.
3) A doping researcher researching the prevalence of doping revealed that based on dialogues with athletes, sports doctors and coaches, "some transfusions" occurred in the Finnish team in 1976, 1980 and 1984 Summer Olympics.
4) Skier 1. In 1985, admitted blood doping earlier the same year. Official inquiry found out that he blood doped also a year before.
5) An official inquiry concluded in 1985 that a handful of blood bags had vanished in a hospital in 1974, 1975 and 1978 for the use of cross-country skiers. The goal of the inquiry wasn't to reveal any names but a few names circulated widely in media likely based on inside leaks of information.
6) Skier 2. In 2001, admitted blood doping "five-six" times during his career (1970-1985).