This is because metal atoms are held together by the electromagnetic attraction from the positive ions to the delocalised electrons.
ðɪs ɪz bɪkˈʌz mˈɛɾəl ˈæɾəmz ɑːɹ hˈɛld təɡˌɛðɚ baɪ ðɪ ɪlˌɛktɹoʊmɐɡnˈɛɾɪk ɐtɹˈækʃən fɹʌmðə pˈɑːzᵻtˌɪv ˈaɪɑːnz tə ðə dᵻlˈɑːkəlˌaɪzd ᵻlˈɛktɹɑːnz.