Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.
Remember, nothing is alleged to--or can have--happened to all of MLB over some one or two seasons: the claim is that PEDs were being used at a slowly but steadily increasing rate (and thus "distorting records") from very roughly 1980 through the present. Were that so, or anything like it, we would expect to see a clear long-term uptrend during this period. But we don't: we see a nearly flat line that, if anything, slopes slightly down. The "boost" just isn't there. But that doesn't seem to stop anyone from talking about it.