Because after all, Germany and Spain are 2 teams that have the highest probability apart from France and England so it is quite natural that in the end one of the winners of this match can make them the champions in this competition, especially seeing from the statistics of the matches they do, it has been seen that these two teams play above the other teams.
They are always big national teams in Europe and last two decades, they won some Euro and World Cup titles. Their national team performances in World Cup and Euro tournaments are not always impressive and in some tournaments, they had to go home early with massive disappointment but generally, their performances are good and it's unrealistic to underestimate their strength.
They are opponents of each other in Euro 2008 and Spain won with a goal of Fernando Torres that is a start of a massive successful cycle of Spain with two Euro titles and one World Cup title within only 6 years.
This will be an interesting big match apart from the France versus Portugal match because looking at what will happen in the quarter-finals this time the France versus Portugal and Spain versus Germany matches will be 2 very interesting matches.
It's similar when we talk about France and Portugal even Portugal is less successful and stable like Germany and Spain last two decades. France and Portugal met each other in Euro 2016 final match in France and the winner is Portugal with a goal from Renato Sanchez who failed to develop his career after that tournament from Bayern Munich to AS Roma.
If France continue to play poorly, they might lose a quarter final match to Portugal that looks to be more united and ambitious.