why do almost all older history books describe him that way?

The problem with a lot of Early Medieval English history is that it was first explored in depth by British 'establishment' intellectuals - starting with F.W. Maitland in the 1780s - who throughout the 19th century used it increasingly as a prop to justify British Imperial policy and cultural supremacy with only a passing interest in exploring external sources or actually interrogating their sources for inherent biases as long as they agreed with their points. At risk of being rather post-structuralist, Victorian historians are very much the 'poster boys' for Barthes' concept of the 'Birth of the Reader' in how we look at texts. As a result of the nature of British university education until the mid-20th century, these ideas formed the core teachings of what we could call "Anglo-Saxon 101" until the 1960s and 1970s, and as a result are still annoyingly prevalent in current pop-history. Perhaps the most famous and pernicious example is the idea of the witanagemot as a proto-Parliament which was used to argue that England was the "mother of democracy" and had a civilising mission to export its own brand of constitutional monarchy across the Empire. Stories such as "Alfred and the Cakes", central to the national pseudo-myth also tend to date from this period.

You may be wondering how this relates to your question: this period of scholarship, broadly speaking, tends to accept historical texts (that aren't obviously allegorical) at face value without interrogating the contexts in which they were produced and proliferated, and the audiences for which they were intended. The death of Rhodri Mawr in 878 was frequently attributed to have occurred at the hands of Alfred of Wessex, for example, simply because he was killed in battle against the English, and thanks to the Anglo-Saxon Chronicle, everybody knows that Mercia was captured by the Vikings in 873 and Ceolwulf II was a Viking pawn. This, of course overlooks contemporary Welsh sources which not only refer to the English explicitly as Anglii, which it typically uses to refer to the Mercians as opposed to the Saxonum of Wessex and claim Ceolwulf's death in the 880s in battle as 'revenge for Rhodri', overlooks significant numismatic evidence that Ceolwulf and Alfred were close allies, crucially overlook that Alfred spent 878 campaigning in South East England and thus couldn't be in Wales, and entirely ignores the provenance of the Anglo-Saxon Chronicle itself.

While typically presented as the Early Medieval English source, the Anglo-Saxon Chronicle survives in a number of regional "recensions" that each provide a somewhat different perspective on events. At its heart, however, the ASC is not the honest national history compiled from regional sources it purports to be, but the family biography of the Cerdicing dynasty which reaches its eventual apotheosis as the royal house of a unified England. Ecgberht is the grandfather of Alfred the great, who first commissioned the Anglo-Saxon Chronicle as a national source. In this context, the extent to which Ecgberht actually exercised political control over England is essential meaningless; its value comes as a rhetorical tool in establishing a prior West Saxon political supremacy, a legitimising precedent to turn what is in reality the expansion of West Saxon borders through conquest of previously-independent polities now under Danish control into a war of liberation of the English People suffering under the "Yoke of the Danes".

What became increasingly clear through scholarship in the late 20th century onwards is that Ecgberht's political control over England was nominative at best, and that he was not able to transform a brief and symbolic overlordship into practical control. Depending on your interpretation of the verb onfeng, it's likely that Wiglaf of Mercia successfully rebelled and retook his throne less than a year after Ecgberht took it, most probably cutting off Ecgberht's army while it was campaigning in Wales and forcing a settlement in order for Ecgberht to return to Wessex unmolested. Similarly, his control of Northumbria was predicated on a symbolic submission rather than a conquest and occupation. Ecgberht's brief London issue coinage is clearly a prestige issue rushed out with an obvious decline in quality from contemporary Mercian issues.

Æthelstan's kingship is, however, based on actual direct political control, thanks largely to the vast establishment of centralised, bureaucratic government authority started by his grandfather, Alfred, and greatly expanded by his father Edward and aunt Æthelflæd through the proliferation of the burghal system and extensive military campaigning, as well as a very real alliance between Mercia and Wessex which had been developing for some 7 decades at this point.

Molyneaux's The Formation of the Kingdom of the English in the Tenth Century is an excellent look at the development of the English state and its mechanisms of power from Edward and Æthelflæd through Æthelstan and onwards.