why only 2 tenses in english?

why do some grammarians claim there are only 2 tenses in english: the simple present and the simple past?

Linguists claim that there are only two "real" tenses because a tense is defined by having an inflected verb. This is only the case in Simple Present and Simple Past. The other ones can be derived by adding "going-to", or "will", or "had". That´s why linguists claim that there are only two tenses. By the way, German has got three "real" tenses.

Well, both notions are possible for these tenses. You often find Past Simple or Simple Past in grammars. In Germany we often call these tenses Simple Past and Simple Present..... I think it´s only a matter of different names describing the same phenomena. Well, not really different names